Quote:
Originally Posted by TDiTP_
Conclusions:
Tested decoders can be divided into two groups:
"A" - liba52, azid, sfse, cyberlink.
"B" - libav, nero7, nero9, sonic, arcsoft. UPD: internal decoder of "SurCode for Dolby Digital® 5.1 Encoder and Decoder" was tested. It's belongs to "B".
1). Frequency response of "A" is closer to source than frequency response of "B" (downturn of high frequencies, usually after 13-17 kHz). It's very strange for a predetermined process!
2). There are differences in SNR between decoders (and between groups) but in general they can be considered negligible (i think).
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Thank you for doing these tests. They are very thorough. For the AC-3 test I highly suspect that the difference in frequency response is due to dithering of 0-bit mantissas. This is one part of the decoding process that is not narrowly defined in the specification. It says that "any reasonably random sequence may be used" and that the range can be anywhere from +/- 0.5 to +/- 0.75. High frequency coefficients are more likely to be quantized to 0 bits and therefore to require dithering in the decoder.
Also, the internal representation vs. the output sample format (and the conversion process) could be different for the decoders. For example, some decoders may use a 16-bit or 24-bit fixed point representation in the decoder, while others may use floating-point then convert to 24-bit.