Network device and balancing method used in the network device

Abstract

The present application discloses an on-board / co-packaged optical device with transmit-side equalization. Transmit-side equalization for a network device and a network communication method employing an on-board / co-packaged optical device is disclosed. An illustrative network device includes a substrate having a host device IC and an optical module IC connected by a short-distance link. The optical module IC with a transmit chain includes a CTLE to at least partially compensate for the channel response of the short-distance link, and a driver for a photoelectric transmitter coupled to an optical fiber to amplify the CTLE output. The host device IC includes: a parallel-to-serial converter, the parallel-to-serial converter generates a digital codeword stream; a digital-to-analog converter, the digital-to-analog converter provides an analog signal to the short-distance link; and a pre-equalizer, the pre-equalizer couples the parallel-to-serial converter to the digital-to-analog converter, and the pre-equalizer filters the digital codeword stream to at least partially compensate for the channel response of a combined channel including the short-distance link, the CTLE, the driver, and the photoelectric transmitter.

Description

Background Art

[0001] The Institute of Electrical and Electronics Engineers (IEEE) Standards Association has published an IEEE standard for Ethernet, namely IEEE Standard 802.3-2015, which will be familiar to those of ordinary skill in the art to which this application belongs. The standard provides general medium access control specifications for local area networks (LANs) operating at selected speeds from 1 Mb / s to 100 Gb / s over coaxial cables, twisted pair cables, fiber optic cables, and electronic backplanes with various channel signal constellations. With the continued demand for higher and higher data rates, even if equalizers are forced to operate at faster symbol rates, extensions to the standard must also take into account the increase in channel attenuation and dispersion.

[0002] At symbol rates approaching or exceeding 100 Gsym / s (Gigasymbols per second), even the printed circuit board traces connecting the optical signaling modules to the packaged integrated circuits can become a significant source of signal loss. For network equipment, one proposed approach to this problem is to minimize or eliminate the printed circuit board traces by locating the optical signaling modules very close ("onboard"), or even on the same package substrate ("co-package") as the integrated circuit devices providing the network communication functions. In this context, the Optical Internet Forum has proposed a general electrical I / O standard "OIF-CEI-05.0 Clause 26: CEI-112G-XSR-PAM4 Ultra-Short Reach Interface," which is incorporated herein by reference. The proposed standard limits the trace length to 50 mm (less than 2 inches).

[0003] The use of on-board / co-packaged optics is intended to reduce the power consumption associated with high symbol rate I / O data streams, but further reductions in this power consumption are still desired to offset increased component density. Summary of the invention

[0004] Accordingly, a network device and a network communication method using on-board / co-packaged optical devices with transmit-side equalization are disclosed herein, which can improve performance and efficiency by reducing power requirements for equalization. An illustrative network device includes a substrate having a host device IC (integrated circuit) and an optical module IC connected via a short-distance link. The optical module IC with a transmit chain includes a CTLE (continuous time linear equalizer) to at least partially compensate for the channel response of the short-distance link, and a driver for an optoelectronic transmitter that amplifies the CTLE output, and the optoelectronic transmitter is coupled to an optical fiber. The host device IC includes: a parallel-to-serial converter that produces a digital codeword stream; a digital-to-analog converter that provides an analog signal to the short-distance link; and a pre-equalizer that couples the parallel-to-serial converter to the digital-to-analog converter, and the pre-equalizer filters the digital codeword stream to at least partially compensate for the channel response of the combined channel including the short-distance link, the CTLE, the driver, and the optoelectronic transmitter. An associated method includes filtering a digital symbol stream using a pre-equalizer to at least partially compensate for a channel response of a combined channel including a short reach link, a CTLE, a driver, and an optoelectronic transmitter.

[0005] Another illustrative network device includes a substrate having a host device IC (integrated circuit) and an optical module IC connected via a short-distance link. The optical module IC with a transmit chain includes: a clock and data recovery circuit that obtains a digital codeword stream via the short-distance link; a pre-equalizer that filters the digital codeword stream to at least partially compensate for the channel response of a combined channel; and a driver for an optoelectronic transmitter that amplifies the filtered signal, and the optoelectronic transmitter is coupled to an optical fiber. The combined channel includes a driver, an optoelectronic transmitter, an optical fiber, and a remote photodetector. An associated method includes filtering the digital codeword stream using a pre-equalizer to at least partially compensate for the channel response of the combined channel, wherein the combined channel includes at least a driver, an optoelectronic transmitter, an optical fiber, and a remote photodetector.

[0006] Each of the foregoing embodiments may be implemented individually or in combination, and may be implemented using any one or more of the following features in any suitable combination: 1. The combined channel further includes an optical fiber and a remote photodetector. 2. The combined channel further includes an optical fiber, a remote optical module IC, and a remote short-distance link, the remote short-distance link connecting the remote optical module IC to the remote host device IC. 3. The pre-equalizer is a finite impulse response filter with one or more floating taps. 4. The optical module IC also has a receive chain, which includes: a photodetector for converting an optical receive signal into an electrical receive signal; and a gain-controlled amplifier, which transmits the electrical receive signal to the host device IC via the short-distance link. 5. The host device IC further includes: an analog-to-digital converter, which converts the electrical receive signal into a digital receive signal; an equalizer, which operates on the digital receive signal to produce an equalized signal; and a data recovery element, which converts the equalized signal into a received digital data stream. 6. The host device IC further includes a training controller, which modifies the coefficients of the pre-equalizer and the equalizer to better compensate for the channel response of the combined channel. 7. The training controller provides a training frame stream with header information, the header information including coefficient updates for the remote pre-equalizer. 8. The optical module IC also has a receiving chain, which includes: a local photodetector, which is used to convert an optical receive signal into an electrical receive signal; an analog-to-digital converter, which is coupled to the local photodetector to derive a digital receive signal; an equalizer, which operates on the digital receive signal to produce an equalized signal; and a data recovery element, which converts the equalized signal into a received digital data stream. 9. A training controller that modifies the coefficients of the pre-equalizer and the equalizer to better compensate for the channel response of the combined channel. 10. The training controller provides a training frame stream with header information, the header information including coefficient updates for the remote pre-equalizer. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 An illustrative communication network is shown.

[0008] Figure 2 is a perspective view of an illustrative switch with co-packaged optics.

[0009] Figure 3 is a schematic diagram of an illustrative package substrate with a host device IC and an optical module IC.

[0010] Figure 4 is a block diagram of an optical module IC coupled to a host device IC in a first configuration.

[0011] Figure 5is a block diagram of an optical module IC coupled to a host device IC in an alternative configuration.

[0012] Figure 6 is a block diagram of an illustrative pre-equalizer. DETAILED DESCRIPTION

[0013] Although specific embodiments are presented in the drawings and the following description, it is to be kept in mind that they do not limit the present disclosure. Rather, they provide a basis for one of ordinary skill in the art to identify alternatives, equivalents, and modifications that are within the scope of the appended claims.

[0014] For context, Figure 1 An illustrative communication network, such as might be found in a data processing center, is shown having a plurality of server racks 102-106, each server rack containing a plurality of servers 110 and one or more switches 112. The switches 112 may be referred to as "top of rack" (TOR) switches, each of which is connected with fiber optic cables 120 to one or more aggregator switches 114 for interconnection and connection to a regional network and the Internet.

[0015] Switches and servers are just two examples of network devices that may employ on-board or co-packaged optics for high-speed communication links via fiber optic cable 120. Other examples include routers, hubs, bridges, and network attached storage devices.

[0016] Figure 2 An illustrative TOR switch 112 with co-packaged optical devices is shown. The switch 112 includes an application specific integrated circuit (ASIC) as its host device IC, providing packet switching connections between its various input / output (I / O) ports. In the co-packaged configuration, a multi-chip module (MCM) package 202 includes a host device on a package substrate that also carries multiple optical module ICs. Optical fibers 204 couple each of the front panel couplers 206 to a corresponding one of the optical module ICs. Illustrative front panel couplers include multi-fiber push-on (MPO) connectors and multi-fiber terminal push-on (MTP) connectors. In an alternative on-board configuration, a printed circuit board mounts the optical module IC near the host device IC (e.g., within 50 mm). In either configuration, the distance between the host device IC port and the optical fiber is minimized to reduce attenuation of high-frequency communication signal components.

[0017] Figure 3An illustrative MCM package substrate 301 is shown having a switch ASIC 302 at the center and an array of optical module ICs 304 around the periphery. Each optical module IC 304 includes a connector 306 for coupling an optical fiber 308 to an integrated phototransmitter and photodetector. The switch ASIC 302 shown provides packet switching connections between its 64 electrical I / O ports. Each I / O port can support multi-channel input data streams and multi-channel output data streams, with each channel transmitting 4-level pulse amplitude modulation (PAM4) symbols at up to 58 Gsym / s or more. As shown in FIG. Figure 4-Figure 5 As shown, a photoemitter is coupled to each output channel and a photodetector is coupled to each input channel.

[0018] Figure 4 A first configuration of one input (receive) data flow channel and one output (transmit) data flow channel is shown. Although components are shown and described herein using functional blocks, it should be understood that these blocks represent integrated circuit modules commercially available in component libraries of various application-specific integrated circuit design software packages and / or as firmware executed by a programmable controller or processing unit.

[0019] Each I / O port of the switch ASIC 302 may be coupled to one or more channels as needed to support the desired data rate. For each channel, the switch ASIC provides a transmit buffer 402 to buffer digital transmit data, which is typically received in the form of blocks of bits or multi-bit symbols. In this example, a parallel to serial converter 404 accesses the buffer to produce a serialized stream of PAM4 symbols. Other example implementations may produce a stream of bits or other symbols. The symbol stream preferably conforms to a standard protocol including frames with alignment markers, such as the standard protocol for PCS data streams set forth in the Ethernet standard (IEEE Standard 802.3-2015).

[0020] An optional finite impulse response (FIR) filter 406 may be used to provide pre-emphasis of high frequency components to counteract attenuation of the short-distance link 410. A digital-to-analog converter (DAC) converts the filtered signal to analog form, and a driver 408 provides sufficient current to transmit the transmit signal to the optical module IC 304 across the short-distance link 410. The short-distance link 410 consists of conductive traces on a package substrate (co-packaged optics) or a printed circuit board substrate (on-board optics). Although these traces may be limited to, for example, less than 50 mm, they may still cause significant frequency-dependent attenuation and dispersion in the upper range of the signal frequencies contemplated herein.

[0021] In the optical module IC 304, a continuous time linear equalizer (CTLE) filter 412 cooperates with an optional FIR filter 406 to compensate for the effects of a short-distance link 410. A clock and data recovery (CDR) module 414 operates on the filtered signal to derive a clock signal for sampling and re-digitizing the digital symbol stream. A pre-equalizer 416 operates on the digital symbol stream to at least partially compensate for the combined channel effects of the components of the optoelectronic transmitter, the driver for the optoelectronic transmitter, the optical fiber, and the remote receiver. A DAC 420 converts the pre-equalized signal to an analog signal, which is amplified by a driver 422 for the optoelectronic transmitter to drive an optoelectronic transmitter 424. The optical fiber connector 306 couples the optical signal from the optoelectronic transmitter to the optical fiber for transmission to a remote photodetector and receiver.

[0022] Here we note that pre-equalization generally increases the amount of signal energy dissipated within the channel by increasing the signal energy in those frequency ranges where the channel signal attenuation is highest, in addition to the necessary power consumption of the pre-equalizer 416 itself. However, for the contemplated channels, this increased power consumption is small compared to the achievable power savings resulting from the associated reduction in receiver complexity, and especially compared to the power savings that can be achieved through equalizer design.

[0023] In the optical module IC 304, the fiber connector 304 further couples the optical fiber to an associated photodetector 430, which converts the optical receive signal into electrical form, typically a receive signal current that can be converted to a receive signal voltage by a transimpedance amplifier (TIA). A gain control amplifier 432 applies an adjustable gain to optimize the receive signal range for processing by subsequent components of the receive chain. A CTLE filter 434 provides anti-aliasing and optional spectrum shaping before the signal is digitized by an analog-to-digital converter 436. A digital equalizer 438 provides adaptive equalization, working with a remote pre-equalizer to compensate for channel effects. A CDR module 440 derives a sampling clock signal and recovers the receive symbol stream from the equalized signal.

[0024] The optical module IC 304 further includes an optional FIR filter 442 to provide pre-emphasis of high frequency components to counteract the attenuation of the short-distance link 446. A digital-to-analog converter (DAC) converts the filtered signal to analog form, and a driver 444 provides sufficient current to transmit the transmit signal to the host device IC 302 across the short-distance link 446, wherein a CTLE filter 448 is combined with the optional FIR filter 442 to compensate for the effects of the short-distance link 446. A clock and data recovery (CDR) module 450 operates on the filtered signal to derive a simple clock signal for recovering the received symbol stream. A serial-to-parallel converter 452 stores blocks of the received symbol stream in a receive buffer 454, enabling the core circuitry of the host device IC to use a lower clock rate.

[0025] The optical module IC 304 further includes a training control module 418 to adapt the coefficients of the pre-equalizer module 416 and / or the equalizer 438. The training control module 418 can generate a training frame sequence that replaces the transmitted data stream during the duration of the link training phase, and can detect equalization errors associated with the training frame symbols in the received data stream. Combining the equalization errors with the training frame symbols, the training controller 418 can adapt the coefficients of the remote pre-equalizer and / or the local equalizer 438 to optimize their combined compensation for the combined reception channel effects. The training controller 418 can transmit the remote pre-equalizer coefficient updates via various reverse channel options including, for example, a dedicated field in the training frame of the transmitted codeword stream. Accordingly, the training controller 418 can extract updates for the local pre-equalizer 416 from a dedicated field in the frame of the received codeword stream. For further details on one illustrative example, see, for example, commonly owned U.S. Patent 10,212,260, "SerDes Architecture with a Hidden Backchannel Protocol," which is incorporated herein by reference.

[0026] Although Figure 4 The configuration shown minimizes the complexity of the I / O circuitry on the host device IC 302, but we note here that with a slight increase in complexity, the overall link efficiency can be significantly improved, as shown in FIG. Figure 5 The illustrative configuration shown provides one in which the components required in optical module 304 are significantly reduced.

[0027] exist Figure 54, the host device IC includes a transmit buffer 402 and a parallel to serial converter 404, a DAC and a driver 408 for a short reach link 410, but the FIR filter 406 is omitted in favor of a pre-equalizer 506. The transmit chain of the optical module 304 includes only a CTLE filter 412, an opto-emitter driver 422, and an opto-emitter 424. As before, the fiber optic connector 304 couples the opto-emitter to the transmit fiber and the receive fiber to the photodetector 430. Similar to the transmit chain, the receive chain of the optical module is minimized to include only the photodetector 430 and a gain control amplifier 432. A short reach link 446 couples the optical module IC 304 to the host device IC 302. The host device IC 302 includes a CTLE filter 434, an analog-to-digital converter 436 to digitize the received signal, an equalizer 438, a clock and data recovery module 440, a serial to parallel converter 452, and a receive buffer 454. A training control module 414 is included in the host device IC 302 to adapt the coefficients of the pre-equalizer 506 and the equalizer 538 .

[0028] The pre-equalizer 506 filters the transmit symbol stream to at least partially compensate for the response of the combined channel, which includes the short-reach link 410, the CTLE 412, the driver 422, the optoelectronic transmitter 424, the optical fiber channel, the remote optoelectronic detector, the remote gain control amplifier, and the remote (receive) short-reach channel. The CTLE 412 and the receive equalizer 538 can help compensate for the effects of the combined channel, but preferably, most of the compensation is performed by the pre-equalizer 506. Since the symbols of the transmit data stream can be represented using only one or two bits, the pre-equalizer can be implemented without multipliers, which greatly reduces the power requirements relative to comparable receive-side equalizers.

[0029] Since the pre-equalizer 416 ( Figure 4 ) or pre-equalizer 506 ( Figure 5 ) reduces the execution requirements of equalizer 438, equalizer 538, so the equalizer can be implemented with lower complexity. For example, the number of coefficients and bit resolution can be reduced. A decision feedback equalizer with feedback and feedforward filters is not required, and the required equalizer execution can be achieved without feedback (eliminating the feedback filter) or without a feedforward filter. In some cases, the CTLE filter 434 can provide sufficient receive-side equalization so that the equalizer 438, equalizer 538 can be completely eliminated.

[0030] We note here that the various components of the combined channel may lengthen the combined channel response, especially if there are any significant reflections, such as might occur with a short-reach link. To effectively combat this lengthening, the pre-equalizer may optionally employ a Figure 6The movable taps are shown. The transmitted code stream is Figure 6 The left side is represented by X k-n , where the subscript represents the time index, k is the index of the current codeword, and n is the offset value. Figure 6 Also shown are the filter coefficients C with corresponding multipliers 604. n A set of , which determines the product of the associated filter coefficients and the corresponding codewords from the transmit stream. Figure 6 The first eight filter coefficients C -2 to C5 are fixed - always multiplied by the symbol with the corresponding offset of the coefficient cursor position.

[0031] For each of the remaining coefficients, one of multiplexers 602-1 through 602-M selects between symbols at different possible offsets from the cursor by which the coefficient is to be multiplied. For example, coefficient C6 is multiplied by the output of multiplexer 602-1, which is X. k-6 ,X k-8 ,X k-10 ,…X k-20 The coefficient C7 is multiplied by the output of the multiplexer 602-2, which is X k-7 ,X k-9 ,X k-11 ,…X k-21 A summer 606 determines the sum of the products from the set of multipliers 604, and a latch 608 holds the sum Y. k As the filter output, the filter output of the subsequent interval is calculated simultaneously. The exact number of coefficients, the range of buffered input symbols and the configuration of the multiplexer can be changed. As mentioned previously, the transmission symbol can be represented by one or two bits so that multiplier 604 can be implemented with minimal complexity.

[0032] Numerous alternatives, equivalents, and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the training controller may adjust the pre-equalizer and equalizer coefficients not only during the link training phase, but also during normal use of the communication link. The training controller may further adjust the parameters of the CTLE filter 434 and the gain control amplifier 432. Figure 4 and Figure 5 The order of components shown may be changed, for example, by moving filters from the digital domain to the analog domain.It is intended that the claims be interpreted as covering all such alternatives, equivalents, and modifications as are included within the scope of the appended claims.

Claims

1. A network device, comprising: a substrate having a host device integrated circuit and an optical module integrated circuit connected by a short-range link, The optical module integrated circuit has a transmission chain, and the transmission chain includes: a continuous time linear equalizer that at least partially compensates for a channel response of the short-range link; and a driver for an optoelectronic transmitter, the driver amplifying an output of the continuous-time linear equalizer, the optoelectronic transmitter coupled to an optical fiber; and The host device integrated circuit comprises: a parallel-to-serial converter that produces a stream of digital symbols; a digital-to-analog converter that provides an analog signal to the short-range link; and a pre-equalizer coupling the parallel-to-serial converter to the digital-to-analog converter, the pre-equalizer filtering the digital symbol stream to at least partially compensate for a channel response of a combined channel including the short reach link, the continuous-time linear equalizer, the driver, and the optoelectronic transmitter.

2. The network device according to claim 1, wherein: The combined channel further comprises the optical fiber and a remote photodetector.

3. The network device according to claim 1, wherein: The combined channel further includes the optical fiber, a remote optical module integrated circuit, and a remote short reach link connecting the remote optical module integrated circuit to a remote host device integrated circuit.

4. The network device according to claim 1, wherein: The pre-equalizer is a finite impulse response filter with one or more floating taps.

5. The network device according to claim 1, wherein: The optical module integrated circuit also has a receiving chain, and the receiving chain includes: a photodetector that converts a light reception signal into an electrical reception signal; and A gain controlled amplifier communicates the electrical receive signal to the host device integrated circuit via the short range link.

6. The network device according to claim 5, wherein: The host device integrated circuit further comprises: an analog-to-digital converter, the analog-to-digital converter converting the electrical received signal into a digital received signal; an equalizer that operates on the digital received signal to produce an equalized signal; and A data recovery element converts the equalized signal into a received digital data stream.

7. The network device according to claim 6, wherein: The host device integrated circuit further includes a training controller that modifies coefficients of the pre-equalizer and the equalizer to better compensate for the channel response of the combined channel.

8. The network device according to claim 7, wherein: The training controller provides a stream of training frames with header information including coefficient updates for a remote pre-equalizer.

9. An equalization method for use in a network device, the network device having a host device integrated circuit on a substrate, the host device integrated circuit connected to an optical module integrated circuit on the substrate via a short-distance link, the method comprising: In the optical module integrated circuit: using a continuous time linear equalizer to at least partially compensate for a channel response of the short-range link; as well as amplifying an output of the continuous time linear equalizer using a driver for an optoelectronic transmitter coupled to an optical fiber; as well as In the host device integrated circuit: A digital symbol stream is filtered using a pre-equalizer to at least partially compensate for a channel response of a combined channel including the short reach link, the continuous time linear equalizer, the driver, and the optoelectronic transmitter.

10. The equalization method according to claim 9, wherein: The combined channel further comprises the optical fiber and a remote photodetector.

11. The equalization method according to claim 9, wherein: The combined channel further includes an optical fiber, a remote optical module integrated circuit, and a remote short reach link connecting the remote optical module integrated circuit to a remote host device integrated circuit.

12. The equalization method according to claim 9, wherein: The pre-equalizer is a finite impulse response filter with one or more floating taps.

13. The equalization method according to claim 9, further comprising in the optical module integrated circuit: Converting the optical reception signal into an electrical reception signal; as well as The electrical receive signal is amplified using a gain controlled amplifier for transmission to the host device integrated circuit via the short range link.

14. The equalization method of claim 13, further comprising in the host device integrated circuit: converting the electrical received signal into a digital received signal; operating the digital received signal using an equalizer to produce an equalized signal; as well as The equalized signal is converted into a received digital data stream using a data recovery element.

15. The equalization method according to claim 14, wherein: The host device integrated circuit further includes a training controller that modifies coefficients of the pre-equalizer and the equalizer to better compensate for the channel response of the combined channel.

16. The equalization method according to claim 15, wherein: The training controller provides a stream of training frames with header information including coefficient updates for a remote pre-equalizer.

17. A network device, comprising: a substrate having a host device integrated circuit and an optical module integrated circuit connected by a short-range link, The optical module integrated circuit has a transmission chain, and the transmission chain includes: a clock and data recovery circuit that obtains a stream of digital symbols via the short-range link and via a continuous-time linear equalizer that at least partially compensates for a channel response of the short-range link; a pre-equalizer that filters the digital symbol stream to at least partially compensate for a channel response of a composite channel; and a driver for a photoelectric transmitter, the driver amplifying the filtered signal, the photoelectric transmitter coupled to the optical fiber, Wherein, the combined channel includes the driver, the photoelectric transmitter, the optical fiber and a remote photoelectric detector.

18. The network device according to claim 17, wherein: The optical module integrated circuit also has a receiving chain, and the receiving chain includes: A local photodetector, the local photodetector being used to convert a light receiving signal into an electrical receiving signal; an analog-to-digital converter coupled to the local photodetector to derive a digital received signal; an equalizer that operates on the digital received signal to produce an equalized signal; and A data recovery element converts the equalized signal into a received digital data stream.

19. The network device of claim 18, further comprising a training controller that modifies coefficients of the pre-equalizer and the equalizer to better compensate for the channel response of the composite channel.

20. The network device according to claim 19, wherein: The training controller provides a stream of training frames with header information including coefficient updates for a remote pre-equalizer.

21. An equalization method for use in a network device, the network device having a substrate having a host device integrated circuit and an optical module integrated circuit connected via a short-distance link, the method comprising, in the optical module integrated circuit: obtaining a stream of digital symbols via the short-range link using a clock and data recovery circuit and via a continuous-time linear equalizer that at least partially compensates for a channel response of the short-range link; filtering the digital symbol stream using a pre-equalizer to at least partially compensate for a channel response of a composite channel; as well as amplifying the filtered signal using a driver for a phototransmitter coupled to the optical fiber, Wherein, the combined channel includes the driver, the photoelectric transmitter, the optical fiber and a remote photoelectric detector.

22. The equalization method of claim 21, further comprising in the optical module integrated circuit: Converting the optical reception signal into an electrical reception signal; as well as deriving a digital receive signal from the electrical receive signal; Using an equalizer to operate the digital received signal to generate an equalized signal; as well as The equalized signal is converted into a received digital data stream using a data recovery element.

23. The equalization method of claim 22, further comprising, in the optical module integrated circuit; using a training controller to modify coefficients of the pre-equalizer and the equalizer to better compensate for the channel response of the combined channel.

24. The equalization method according to claim 23, wherein: The training controller provides a stream of training frames with header information including coefficient updates for a remote pre-equalizer.

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