5 ms·

Weird, so manufacturers just haven't implemented uncompressed 6-channel optical audio? Is it bandwidth?

by vortico 9y ago

c2h5oh 9y ago
Uncompressed stereo 16 bit 44.1khz sample rate (cd quality) is <150 KBps so it's definitely not bandwidth.
- makomk 9y ago
  It's bandwidth. More specifically, standard TOSLINK only has the bandwidth to carry stereo audio in uncompressed form. If you want full surround sound it needs to be compressed to fit into the same bandwidth that stereo audio would use.
  - ComputerGuru 9y ago
    I thought the problem wasn’t toslink (the physical optical layer) but rather s/pdif (the software layer) which is constrained by its support for the rca physical layer? IIRC, toslink 1.0 (circa ‘82 or ‘83) had a limit of 2 or 3 mbps while modern toslink is well over 100mbps (constrained more by distance than speed).
    - DiabloD3 9y ago
      SPDIF is not a software layer, but a limited version of AES3 for consumers. In the OSI model, AES3 would be considered layer 2. Layer 2 is not considered software, although it can be performed by software in systems where such a concept makes sense. However, in AES3, this does not make sense, and has always been handled in devices via a chip that merely turns AES3 into I2S to be fed to either a DAC or another interface chip (ex: conversion between AES PHYs is often done by just bridging the receiver of one type to the transmitter of another type semi-blindly via I2S). Also, the AES3 protocol is incredibly simple. It only has a very limited set of flags (very limited in today's sense, more than enough for what was intended in 1985, see https://en.wikipedia.org/wiki/AES3#Channel_status_word https://en.wikipedia.org/wiki/AES3#Channel_status_word ), and was never actually meant to do anything but stereo audio at conventional rates. Doing encoded bitstreams across it is largely a hack, and is limited to the base versions of Dolby and DTS. Using >3mbit over Toslink is not done involving an AES3-purposed PHY, but instead with other unrelated protocols, such as Digital ADAT. Distance on typical Toslink is already constrained to about 30 feet. I have seen other people quote this mythical 100mbit number, but no one has ever been able to cite such a use in the real world: no standard I have ever seen that uses Toslink as a PHY has supported such a transmission rate: ADAT maxes out at around 12mbit, and MADI does not use Toslink at all. Also, in further irony, in use, I have found unbalanced AES3 (such as via RCA or BNC) to be more reliable than Toslink; and virtually all of professional AES3 usage is done by 3 pin XLR because of how unreliable Toslink is... Although a lot of professionals have moved on from AES3 entirely, and use MADI or Dante, or don't use digital transmission pipes inside of their studio at all and run it entirely on a single PC.
      - ComputerGuru 9y ago
        Very interesting reply, thank you. I meant to say “protocol layer” rather than “software layer” but misspoke. The correction is appreciated. I think I’m going to do some more research on that 100mbps number. Perhaps they meant Bd/s?
  - deleted 9y ago
    [deleted]
- ConnorBoyd 9y ago
  CD quality is 1.4 mbps 16 * 44,100 * 2 = 1,411,200
  - c2h5oh 9y ago
    Lower case "b" - so bits. 1,411,200bps = 176,400Bps ~~ 172KBps I wasn't that far off.
inopinatus 9y ago
The current standard tops out at a clock of 6.144MHz which is exactly enough to carry 24-bit 48Khz 2-channel uncompressed audio in a 32-bit, differential Manchester encoded frame that includes sync & signalling bits. Running AC3 and DTS over this is a bit of a hack, but it's a standardised hack. HDMI supersedes S/PDIF so comprehensively that the S/PDIF standard will likely never be substantially revised. There's no compelling reason to invest in the R&D.

Weird, so manufacturers just haven't implemented uncompressed 6-channel optical audio? Is it bandwidth?

c2h5oh 9y ago

Uncompressed stereo 16 bit 44.1khz sample rate (cd quality) is <150 KBps so it's definitely not bandwidth.

makomk 9y ago
It's bandwidth. More specifically, standard TOSLINK only has the bandwidth to carry stereo audio in uncompressed form. If you want full surround sound it needs to be compressed to fit into the same bandwidth that stereo audio would use.
- ComputerGuru 9y ago
  I thought the problem wasn’t toslink (the physical optical layer) but rather s/pdif (the software layer) which is constrained by its support for the rca physical layer? IIRC, toslink 1.0 (circa ‘82 or ‘83) had a limit of 2 or 3 mbps while modern toslink is well over 100mbps (constrained more by distance than speed).
  - DiabloD3 9y ago
    SPDIF is not a software layer, but a limited version of AES3 for consumers. In the OSI model, AES3 would be considered layer 2. Layer 2 is not considered software, although it can be performed by software in systems where such a concept makes sense. However, in AES3, this does not make sense, and has always been handled in devices via a chip that merely turns AES3 into I2S to be fed to either a DAC or another interface chip (ex: conversion between AES PHYs is often done by just bridging the receiver of one type to the transmitter of another type semi-blindly via I2S). Also, the AES3 protocol is incredibly simple. It only has a very limited set of flags (very limited in today's sense, more than enough for what was intended in 1985, see https://en.wikipedia.org/wiki/AES3#Channel_status_word https://en.wikipedia.org/wiki/AES3#Channel_status_word ), and was never actually meant to do anything but stereo audio at conventional rates. Doing encoded bitstreams across it is largely a hack, and is limited to the base versions of Dolby and DTS. Using >3mbit over Toslink is not done involving an AES3-purposed PHY, but instead with other unrelated protocols, such as Digital ADAT. Distance on typical Toslink is already constrained to about 30 feet. I have seen other people quote this mythical 100mbit number, but no one has ever been able to cite such a use in the real world: no standard I have ever seen that uses Toslink as a PHY has supported such a transmission rate: ADAT maxes out at around 12mbit, and MADI does not use Toslink at all. Also, in further irony, in use, I have found unbalanced AES3 (such as via RCA or BNC) to be more reliable than Toslink; and virtually all of professional AES3 usage is done by 3 pin XLR because of how unreliable Toslink is... Although a lot of professionals have moved on from AES3 entirely, and use MADI or Dante, or don't use digital transmission pipes inside of their studio at all and run it entirely on a single PC.
    - ComputerGuru 9y ago
      Very interesting reply, thank you. I meant to say “protocol layer” rather than “software layer” but misspoke. The correction is appreciated. I think I’m going to do some more research on that 100mbps number. Perhaps they meant Bd/s?
- deleted 9y ago
  [deleted]
ConnorBoyd 9y ago
CD quality is 1.4 mbps 16 * 44,100 * 2 = 1,411,200
- c2h5oh 9y ago
  Lower case "b" - so bits. 1,411,200bps = 176,400Bps ~~ 172KBps I wasn't that far off.

inopinatus 9y ago

The current standard tops out at a clock of 6.144MHz which is exactly enough to carry 24-bit 48Khz 2-channel uncompressed audio in a 32-bit, differential Manchester encoded frame that includes sync & signalling bits. Running AC3 and DTS over this is a bit of a hack, but it's a standardised hack. HDMI supersedes S/PDIF so comprehensively that the S/PDIF standard will likely never be substantially revised. There's no compelling reason to invest in the R&D.