A question on SPDIF

Bronze · 9th December 2004, 12:54 AM

How does AC3 maintain its' 32 - 640kbps datarate over SPDIF? I can only think of two ways that it does this:

1) The SPDIF interface is run at the AC3 data rate. However this would mean the first block of audio could not be decoded until at least 20ms after the arrival of the first bit which is at the threshold of human delay sensitivity from what I've read.

2)SPDIF is run 6.144, 5.6448 or 4.096 MHz but just transmits frames whenever it has data, so the reciever can't require back-to-back frames. I'm guessing this is what the AC3 transmitters do. They burst an entire syncframe and then wait the average amount of audio time in each frame. However this requires that the transmitter 'knows' the nominal bitrate of the stream. Is this the way it works, or do the decoders have microseconds to do their thing?

gmarsh · 9th December 2004, 02:57 PM

For the pure tech details, get your hands on the IEC61937 spec.

AC-3, DTS, etc are sent in bursts of data. Basically the SPDIF stream says "hey, there's a hunk of data coming, listen to the next <...> frames, and ignore the rest until I tell you there's another hunk coming"

9th December 2004, 12:54 AM	#1
Bronze diyAudio Member Join Date: Oct 2002 Location: NY Capital Region	A question on SPDIF How does AC3 maintain its' 32 - 640kbps datarate over SPDIF? I can only think of two ways that it does this: 1) The SPDIF interface is run at the AC3 data rate. However this would mean the first block of audio could not be decoded until at least 20ms after the arrival of the first bit which is at the threshold of human delay sensitivity from what I've read. 2)SPDIF is run 6.144, 5.6448 or 4.096 MHz but just transmits frames whenever it has data, so the reciever can't require back-to-back frames. I'm guessing this is what the AC3 transmitters do. They burst an entire syncframe and then wait the average amount of audio time in each frame. However this requires that the transmitter 'knows' the nominal bitrate of the stream. Is this the way it works, or do the decoders have microseconds to do their thing?

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9th December 2004, 02:57 PM	#2
gmarsh diyAudio Member Join Date: Apr 2004 Location: Halifax, NS, Canada	For the pure tech details, get your hands on the IEC61937 spec. AC-3, DTS, etc are sent in bursts of data. Basically the SPDIF stream says "hey, there's a hunk of data coming, listen to the next <...> frames, and ignore the rest until I tell you there's another hunk coming"