RE: Audio Digital Data Streams

["James Hoye" <james.hoye@xxxxxxx>]

    Is all the info necessary to define the exact format of the data encoding always transmitted in an audio digital data stream or is it sometimes ambiguous ?

    Specifically with ITV Digital (assuming it is still around by the time you read this), do they ever transmit anything apart from standard stereo or Dolby Surround ? (I heard not as they don't have the bandwidth). When they do transmit Dolby Surround is there anything in the data stream so that you know it is in surround rather than ordinary stereo ? I do have a digital output from my Nokia ex OD box.

 

AFAIK Dolby Surround is matrixed onto the stereo channels by phase shifting.  This would mean that ANY stereo signal (analogue, AC3, PCM) could carry this extra information (centre and rear mono).  The processor (there's that word again) will do the necessary maths to be able to create these two pseudo-channels.  This is why the frequency response of Dolby Surround is so poor compared to Dolby Digital (AC3) which it a digital bitstream containing data for a number of DISCRETE audio channels.

 

Digital output on you OD box merely outputs a stereo PCM signal so that you can feed it to an AV amp (or other DAC) for converting to analogue and amplification.  As stated above, the stereo channels COULD contain matrixed audio, but AFAIK PCM doesn't carry any flags to show that anything other than bog-standard stereo (or perhaps mono) is being transmitted.

 

HTH, James H

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