Wha't's the optical audio output of the ATV?

Discussion in 'Apple TV and Home Theater' started by Duffinator, Sep 8, 2007.

  1. Duffinator macrumors 6502

    Duffinator

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    Sep 3, 2007
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    130 miles NE of Cupertino
    #1
    I was discussing in another thread what the ATV outputs via optical. I have my ATV connected via optical to my Denon 3805. With other sources I have this provides a digital input and the receiver decodes the signal and then you have stereo, DD, DTS, etc. My receiver shows a PCM Dig input signal from the ATV. The question I have is the ATV decoding the MP3 and AAC files into stereo? I'm pretty sure my receiver doesn't decode MP3 and I'm positive it doesn't decode AAC files so what's going on?

    Thanks for any insight you can provide.
     
  2. SthrnCmfrtr macrumors 6502

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    Las Vegas, NV
    #2
    PCM 2-channel WAV, or something like that. Just sound.

    It's just outputting the same thing you'd get from a headphone jack, it's just that it's completely unmodified and unadamaged from life as an analog signal. So you can get DTS 5.1 channel albums (like Dark Side of the Moon), rip them as Apple Lossless, and then play them in iTunes and get beautiful 5.1 channel sound. Through an analog connection, though, due to what the uneducated view as the "warmth" of analog, you'd just get a bunch of white noise :)

    Sorry if I'm misunderstanding you.
     
  3. Duffinator thread starter macrumors 6502

    Duffinator

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    #3
    Thanks. So is the iTunes software or the ATV converting the files to PCM?
     
  4. SthrnCmfrtr macrumors 6502

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    #4
    The ATV hardware, I presume.
     
  5. lostless macrumors 6502

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    Oct 22, 2005
    #5
    An optical output only outputs 1 of 3 formats. PCM, or uncompressed stereo signal. AC3 which is dolby digital, compressed format that is capable of 5.1. DTS is the third. Like dolby digital, is compressed, but is a much more efficient codec. But a computer, ATV, ect. do not send sounds in their native format. you cant hear compressed data. they are always converted to PCM. It not a conversion per say, but PCM is the only pure digital representation before converting to analog. Computers just play sounds and the sounds have to be in PCM to be converted to analog. Its hard to explain. Its all done under hood and instantly done by the OS/sound card. Like a dolby digital signal to a receiver. The decoding to PCM is instant before it converts to analog. Its the reason you can use audio hijack to rip any audio from your computer. It just reads the pcm output of program and saves it as an AIFF, which is just a PCM container like WAV.
    As for the apple tv, since sending mp3 or aac audio streams to a audio receiver is usless, PCM is the uncompressed, near analog, representation of the mp3s/aacs. compressed audio do not contain pure waveform representations.
     
  6. SthrnCmfrtr macrumors 6502

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    #6
    Thanks for clarifying :) I thought it was something like that, but didn't want to talk out of my butt.
     
  7. Duffinator thread starter macrumors 6502

    Duffinator

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    #7
    Thanks for clearing that up for me. I'm following all of this. Now is it the ATV or iTunes, Windows XP, or something else on my PC concerting the signal to PCM? I'm still unclear on that.

    The signal from my ATV sounds light years better than my Media MVP since I'm using the fine DAC's in my Denon. But the bass is slightly boomy which is not the case with CD's or SACD's from my other players. So if the PC is doing the conversion maybe I can tweak the settings on my audio card to correct the bass.
     
  8. lostless macrumors 6502

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    Oct 22, 2005
    #8
    Its whatever program/device is trying send audio to a sound card to be converted analog. The AYV and either a. send an uncompressed PCM through a digital output, or convert to analog to use analog out ports the back. In fact it does both at the same time. So the apple TV is reading the music in MP3/aac and then playing it back as PCM to be converted to analog via itself or using your stereos analog to digital coverter (like a cd, a cd holds PCM data that needs to be converted to analog. We cant hear digital ;) ). It not really a conversion, but an uncompressed representation of whatever audio format is being played. hard to explain.
     
  9. Avatar74 macrumors 65816

    Avatar74

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    Feb 5, 2007
    #9
    The AppleTV converts the content on the fly from its existing format, be it MP3, AAC or 24-bit AIFF, to two-channel (stereo) 16-bit, 44.1kHz, Linear PCM. It is useful to note that while PCM is not considered a compression schema, it is a mathematically limited representation of the analog waveform. 16-bit samples have a finite Dynamic Range (amplitude) and 44.1kHz sampling frequency is at the Nyquist limit which has a finite set of quantizaton intervals and therefore amplitude values are subject to some degree of quantization error and frequency values are subject to frequency response roll off as the sampled frequency approaches 1/2 the Nyquist limit (i.e. 22.05 kHz).

    However, most modern D/A converters employ sample & hold buffers large enough to allow internal reclocking of the signal and during recording and/or mastering, a low-pass filter is applied at 1/2 the Nyquist limit to avoid introduction of frequencies that would induce aliasing during reconstruction (it is a common myth that aliased frequencies are the result of digital encoding whereas it is actually an artifact resulting from a faulty signal reconstruction).

    The bottom line is that provided you use a near-lossless encoding schema, e.g. 128 Kbps MPEG-4 AAC (determined by the AES to be perceptiually indiscernible from 16-bit LPCM), you'll mitigate the potential for reconstruction errors and artifacts, and you should not be able to discern a difference between that playback and playback from the original 16-bit LPCM source from which the AAC was transcoded.

    Yes. I know what hardcore audiophiles will say to that, and they're wrong.

    P.S. With regard to lostless's reply, DTS by the way is not more efficient than Dolby Digital. Theatrical DTS uses a 1.5Mbps ADPCM bitstream whereas theatrical Dolby Digital uses a 640 Kbps bitstream coupled with perceptual algorithms and bandpass filtering to achieve the same or better result. Therefore, Dolby Digital is more efficient than DTS. DTS is regarded by some as capable of a more accurate reproduction but that claim has yet to be substantiated in double-blind testing.
     
  10. Squonk macrumors 65816

    Squonk

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    Mar 15, 2005
    #10
    I hope that when the ATV gets an update that they change the optical out to have 5.1 surround sound. Of course, that would require all of the source material on the iTunes store to be 5.1, right? I cannot see buying an ATV until it will do 5.1. Until then, I'll use my DVD player and my DVR which give me surround.
     
  11. Avatar74 macrumors 65816

    Avatar74

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    Feb 5, 2007
    #11
    You do realize that Dolby Surround is still present in the two-channel output. Granted this isn't the same as Dolby Digital, but it's an option.

    Frankly I still find the AppleTV useful because 90 percent of my use of it is as a music bridge. While I have a rather substantial surround system, I still find the convenience of movies on the AppleTV menu too great to pass up. I don't need to watch every single movie in Dolby Digital 5.1 surround. And there's a ton of older films and television shows that were never mastered to 5.1-channel surround.
     

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