Re: PCM vs. Bitstream (Denon 3808 and Panasonic BD30)
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Originally Posted by DrRoebuck
So is there any validity to what so many others have said, regarding PCM being better than Bitstream?
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Yes and no.
LPCM signals allow you to use an 'old' HDMI receiver that doesn't incorporate its own decoding for HD audio streams. This saves people with older kit from replacing their receiver (something which can be very expensive) and it helps keep prices of HDMI kit in the second hand market at a reasonable level.
The
quid pro quo with this is that you need a player which is itself capable of decoding all of the relevant audio formats and streaming them as LPCM across an HDMI interface - this means the PS3 and not much else.
But in strict quality terms, the issue goes deeper. Audio signals transmitted across an HDMI interface do not have their own clock signal to regulate the data flow. As a result of this, audio sent across HDMI is suceptible to high levels of transmission jitter. Ordinarily, transmission jitter can easily be filtered out with a Phase Locked Loop (although this does not solve all problems), but with HDMI the cost of doing this gets expensive.
Where does this leave us? Audio signals across HDMI are open to both transmission and conversion jitter, and there are a wealth of lab tests on the web to illustrate what this means for signal fidelity. As any Acoustician will tell you - just as this one is - broadband jitter spectra equates to a reduction in SNR whereas periodic jitter equates to frequency depenent distortions. Audio signals transmitted across an HDMI interface are thus inherently proned to reduction in total Signal to Noise ratio as a consequence of high levels of broadband jitter.
What does this mean for HD Audio? LPCM is arguably more subjectively open to 'corruption' of the audio signal when transmitted across an HDMI signal since it is not 'packed' data and any conversion errors introduced will impact directly upon signal fidelity. 'Packed' data, that is to say, bitstream of Dolby True HD or DTS HD Master Audio, could be said to be less vulnerable to direct signal corruption
because it is yet to be decoded when it reaches the receiver.
Is there any real world difference, however? That's where subjectivity comes into play. The worst that could happen to an LPCM signal sent over HDMI would be a lessening of the SNR - is that a deal breaker? For me it is not, at least, not in multichannel. Receiver/Processor manufacturers will of course be happy to support the rumour that the quality of the decoding matters - which is a fallacy. As with all things in this wonderful hobby, only the individual can decide.
