Audio Synchronization via Rate Offset Adjustment
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
In packet-switched telephony systems, the asynchronous nature of individual terminals' clock frequencies leads to variable audio delays, causing distracting 'stadium' echo effects due to unsynchronized audio play-outs, which existing methods like absolute time stamping are impractical and costly in terms of signalling overhead.
Innovation Solution
A method where telecommunications terminals transmit rate-indication signals to a server, which calculates and returns an offset value to synchronize the terminals' clock frequencies with a master clock, ensuring consistent audio output rates without requiring significant signalling overhead.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If regular time stamp methods are used to ensure absolute synchronicity, then audio synchronization is improved, but signalling overhead increases significantly
Solution Approach 1:
The invention changes the parameter being controlled from absolute time synchronization to relative rate synchronization. Instead of using time stamps to achieve absolute synchronicity, the system measures the rate at which packets are received and adjusts the playout rate accordingly. This parameter change allows synchronization to be achieved with minimal signalling overhead, as the system only needs to exchange rate information rather than detailed time stamp data.
2Manufacturing precision
If absolute synchronicity is pursued, then audio synchronization is improved, but it becomes impossible to achieve due to acoustic path differences
Solution Approach 1:
The invention inverts the approach to synchronization by abandoning the pursuit of absolute synchronicity and instead focusing on rate matching. Rather than trying to make all terminals output audio at the exact same absolute time, the system ensures that all terminals play back audio at the same rate relative to their own reception timing. This inversion makes synchronization achievable in practical acoustic environments where path differences exist.
3Manufacturing precision
If TDM telephony with common clock source is used, then audio synchronization is improved, but the system becomes incompatible with modern packet-switched networks
Solution Approach 1:
The invention substitutes the mechanical/common clock distribution system of TDM telephony with a software-based rate measurement and adjustment system. Instead of relying on a shared physical clock source that all terminals must synchronize to, the system uses independent packet rate measurements at each terminal and adjusts playout rates accordingly. This substitution enables packet-switched network compatibility while maintaining synchronization precision.
Data Source
AI summary
The audio outputs 29 of each of a plurality of telecommunications terminals in a packet switched system are synchronized by having each terminal transmit a stream of packets whose rate, and therefore duration is determined according to a clock generator 25, and is thus indicative of the rate at which the terminal is generating its audio output. The signals from each terminal are transmitted to a common server. For each terminal, the server uses a master dock to compare the duration of the packet stream with an expected duration, and calculates an offset value which is returned to the respective terminal. Each terminal stores the offset value it receives (20) and uses it to adjust the output of its clock generator 25 so that its operations can be synchronized to the server. This allows all the terminals' digital-to-analog conversion processes to be synchronized such that all their analog outputs are coordinated, allowing co-located acoustic outputs to be synchronous. A second embodiment maintains synchronization by maintaining the volume of the data buffer 23 serving the audio output 29 within predetermined limits.