HOA Signal Compression via Subband Directional Coding
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Solution Overview
Problem
Higher Order Ambisonics (HOA) representations require high bit rates for transmission, making them unsuitable for low-data-rate applications like audio streaming to mobile devices, as existing compression methods typically result in data rates higher than 128 kbit/s.
Innovation Solution
A method for low-bit rate compression of HOA representations involves decomposing the signal into frequency sub-bands, approximating coefficients using a truncated HOA representation and predicted directional sub-band signals, with partial de-correlation of selected coefficient sequences and parametric coding of directional sub-band signals, allowing for efficient encoding and decoding without additional side information.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional HOA compression methods are used, then audio quality is maintained, but data rate exceeds 128 kbit/s
Solution Approach 1:
The HOA signal is divided into multiple frequency subbands, with different coding strategies applied to each subband. Directional signals are separated from ambient signals, and each component is encoded independently with appropriate precision, allowing quality preservation while reducing overall data rate below 128 kbit/s
Solution Approach 2:
Different coding precision is applied to different components of the HOA signal. Directional subband signals use parametric coding with lower precision requirements, while ambient HOA coefficients use perceptual coding with adaptive precision based on their perceptual importance, optimizing the trade-off between quality and data rate
2Measurement precision
If the maximum order N of Spherical Harmonics expansion is increased, then spatial resolution improves, but the number of expansion coefficients grows quadratically
Solution Approach 1:
The full HOA coefficient set is segmented into directional and ambient components. Only the essential directional components are transmitted with full precision, while ambient components use compressed representation, reducing the effective number of coefficients that need to be transmitted while maintaining spatial resolution
Solution Approach 2:
The representation parameters are changed from transmitting all HOA coefficients to transmitting a hybrid representation with directional signals and residual ambient coefficients. This parameter transformation reduces the data volume while preserving the spatial resolution capability of high-order HOA
3Measurement precision
If directional signals are coded with high precision, then spatial accuracy improves, but bit rate increases
Solution Approach 1:
Instead of coding all directional parameters with full precision, the method uses parametric coding that captures the essential spatial characteristics with reduced precision. The parametric representation efficiently encodes direction information using fewer bits while maintaining sufficient spatial accuracy for perceptual purposes
Data Source
AI summary
Encoding of Higher Order Ambisonics (HOA) signals commonly results in high data rates. For data rate reduction, a method (100) for encoding direction information for frames of an input HOA signal comprises determining (s101) active candidate directions (I) among predefined global directions having global direction indices, dividing (s102) the input HOA signal into frequency subbands (II), determining (s103) for each frequency subband active subband directions among the active candidate directions, assigning (s104) a relative direction index to each direction per subband, assembling (s105) direction information for the frame, the direction information comprising the active candidate directions (I), for each subband and each active candidate direction a bit indicating whether or not the active candidate direction is an active subband direction for the respective frequency subband, and for each frequency subband the relative direction indices of active subband directions in the second set of subband directions, and transmitting (s106) the assembled direction information.