Question Please educate me! PCM vs bitstream, ARC, and more

Discussion in 'AV Receivers & Amplifiers' started by RavediggaUK, Jul 9, 2018.

  1. RavediggaUK

    RavediggaUK
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    Hi everyone. New member here!

    I'll save you the loooong story, but I'm trying to troubleshoot some issues between by Samsung KS8000 TV, a Panasonic SA-BT200 5.1 surround system, and two games consoles (PS4 Pro & XBox One X). In doing so I've done a lot of reading online and I'd just like to clarify my understanding of a few features please.

    ARC - Audio Return Channel
    As I understand this, it's a method for a TV to transmit audio via HDMI to a receiver. That's likely very blunt, but correct, yes?
    Am I right in thinking in that case that there needs to be an ARC enabled HDMI port on either end of the signal chain - i.e. output on the TV and input on an audio receiver?

    PCM - pulse code modulation
    This is an output format which is uncompressed, as opposed to bitstream which can carry (for example) a compressed Dolby Digital 5.1 signal.
    PCM is higher quality since it is not compressed (in comparison to bitsream carrying a compressed audio signal at least).
    PCM requires full transit via HDMI, from source to receiver - mediums such as optical are not up to the task of carrying 5.1 or 7.1 audio in PCM format and will limit the signal down to 2-channel stereo.

    Based on the above being correct, if I have a games console which can output 5.1 audio in PCM format over HDMI to a TV and a TV which can catch that PCM signal and then pass the same PCM signal onward to a receiver via their respective ARC HDMI ports, then the receiver should playback the full uncompressed 5.1 audio signal?

    For simple clarity, this is all due to my SA-BT200 surround system now being the weak link in the chain, having no ARC support and only two optical inputs to share between 3 devices (TV and two consoles). I'm looking at my upgrade options and if I've understood the above terms and capabilities correctly, then I think I know what I need to be aiming for as a replacement.

    Cheers! :)
     
  2. gibbsy

    gibbsy
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    ARC (or pain in the arse as I refer to it) has to have the two devices, TV and receiver, having the appropriate HDMI connect. Some TVs however, my Panasonic being one, is not capable to passing through 5.1 (or as I refer to it, another pain in the arse).

    PCM is uncompressed and can pass a full 5.1 signal. However you would need bitstream to pass any HD sources to a receiver from an Xbox or blu ray player to allow DolbyTrue HD or DTS MA HD, which is usually of a better quality than standard Dolby or DTS.

    You could use any modern receiver as a USB hub and gain access to HD sound. There are then a few ways you can connect to the TV, ARC being one. The others are using a separate HDMI to the TV for vision only with a second HDMI going to the receiver for audio although this will depend on any particular player having two HDMI out. The easiest way to connect all devices to the receiver with a single HDMI going to the TV without engaging ARC. Audio from the TV can then be by digital optical to the receiver.
     
  3. andy1249

    andy1249
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    As it stands , ARC has the same capabilities as optical, both carry an spdif signal.
    So if you have a console sending 5.1 PCM to a TV, it will not be able to pass this via ARC to the receiver, only stereo, i.e. same limitations as an optical cable.

    Bitstream means sending an audio signal as is still wrapped in its codec package.
    Standard dolby digital and DTS are lossy and lower quality.
    You can send these to some TVs and have it passed to a receiver via ARC

    Dolby Tru HD and DTS Master audio are also bitstreamed and are lossless and of higher quality.
    These currently cannot be sent via ARC.

    That is the situation right now.
    However eARC is on the way, this will pass anything, but of course you will need all new gear for this.

    Also, last but definitely not least, ARC depends on HDMI CEC, and every manufacturer has their own brand of HDMI CEC.
    This makes ARC unstable and difficult to maintain.
    By far the best connection options are to get a receiver with enough compatible HDMI inputs and connect straight to it.
    Avoid ARC like the plague, as above , its a massive pain.
     
    Last edited: Jul 10, 2018
  4. Mark.Yudkin

    Mark.Yudkin
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    ARC is indeed a method to transfer the audio from the TV back to the audio device when the sound is sourced by the TV. ARC is stable and generally works quite well for most consumers, although some people have had negative experiences, especially with earlier buggy CEC implementations.

    For all of Dolby DD, dts, Dolby TrueHD and dts hd ma, either the source must effect decompression (that is the correct term, decoding is a popular marketing misnomer) to PCM or the sink must effect decompression to PCM for further processing. As such whether you pass the compressed data to the sink (bit-streaming) or whether you pass PCM to the sink is of no consequence and the sound is absolutely identical.

    Two issues affect the above:
    1) S/PDIF was not specified to have the capacity to pass multichannel PCM (optical or coax), so you need to pass Dolby DD or dts, which use lossy compression, to reduce the bandwidth. HDMI (to TV) does not have this limitation, but ARC does. The proposed eARC lifts the restriction.

    2) Dolby Atmos and dts:X are not just compression formats but do include additional spatial metadata. Processing of the metadata of these two object formats are not typically implemented in sources, so that you need to stream these formats to an implementing sink.
     
  5. RavediggaUK

    RavediggaUK
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    Fantastic, thanks very much all for the great advice!

    So... it sounds as though there's not much point in me considering replacing my current Panasonic surround system in favour of one that can take audio via ARC - it simply won't give me the benefits I thought it might. For eARC I'd need all new gear as has been pointed out, TV and everything else, so that's not an option. Though still good to learn.

    Cheers for all of this, it's just what I needed and interesting too.
     
  6. Jamie

    Jamie
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    I think you're getting hung up on ARC unnecessarily.

    As long as an AV receiver had the appropriate HDMI version you require for video you'd be far better running your devices to the AVR to process the audio and pass on the video to the amp.

    AFAIK your TV is 4K with HDR so you will require something fairly modern so it supports the 4K video but that's not a problem.
     

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