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Dolby Digital Question


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20 replies to this topic

#1 OFFLINE   Steve

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Posted 10 April 2010 - 11:27 AM

Does anyone know if Dolby Digital is always recorded, and the system preferences simply determine which audio tracks are selected for output? Or does DD have to be enabled prior to the recording?

Also, for MRV purposes, if DD is always recorded, does it matter what the server setting is, as long as the client is set to Dolby Digital "On"? TIA.
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#2 OFFLINE   MizzouTiger

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Posted 10 April 2010 - 12:02 PM

I believe it's always recorded. My HR20 in the bedroom has DD turned off. My HR20 in the living has DD turned on since I have a surround sound system in there. When using MRV and watching a program recorded on the bedroom HR20 from the living room, the audio is in DD.

So it would appear to me that the DD track is always recorded and the "viewing" DVR determines if the DD track is output or not.
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#3 OFFLINE   Grentz

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Posted 10 April 2010 - 12:12 PM

I have had the same experience as MizzouTiger.
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#4 OFFLINE   Steve

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Posted 10 April 2010 - 01:03 PM

Thanks, guys. That's what I was hoping.
/steve

#5 OFFLINE   David MacLeod

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Posted 10 April 2010 - 01:20 PM

think I had read somewhere here that the dolby on/off only affects playback on that machine.
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#6 OFFLINE   tonyd79

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Posted 10 April 2010 - 02:05 PM

Don't the DVRs just record the signal without conditioning it at all? That would include the DD, right?

Edit to add: I ran a quick test. Recorded a show with DD off, played it back. DD was on.

Edited by tonyd79, 10 April 2010 - 02:11 PM.

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#7 OFFLINE   David MacLeod

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Posted 10 April 2010 - 02:06 PM

iirc that is correct.
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#8 OFFLINE   davring

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Posted 10 April 2010 - 02:09 PM

Don't the DVRs just record the signal without conditioning it at all? That would include the DD, right?


The DVR records the entire data stream.

#9 OFFLINE   TomCat

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Posted 10 April 2010 - 03:26 PM

The DVR records the entire data stream.

That depends. For instance, the "entire" data stream of a tv channel may contain sub channels. Those are demultiplexed out before being sent to the subscribers .

An HD channel uses AC3 as it's audio tracks, which is a variant of DD, so all HD audio is truly DD, usually either 5.1 or 2.0. Although the audio is a separate stream from the video, both are obviously recorded.

(sorry, my first post using an iPad--not quite used to the keyboard yet. Maybe not the best input device for someone as windy as me :D.)
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#10 OFFLINE   davring

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Posted 10 April 2010 - 03:57 PM

The DVR still records all the the sat sends down, maybe not everything the station broadcasts, correct?

I sometimes use my iPhone, smaller keys yet:)

#11 OFFLINE   hilmar2k

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Posted 10 April 2010 - 05:44 PM

The DVR still records all the the sat sends down, maybe not everything the station broadcasts, correct?

I sometimes use my iPhone, smaller keys yet:)


Correct. The stream is recorded as is. Any changes to how it is displayed (either video or audio) has no affect on the recording itself.

#12 OFFLINE   TomCat

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Posted 10 April 2010 - 06:11 PM

The DVR still records all the the sat sends down, maybe not everything the station broadcasts, correct?

I sometimes use my iPhone, smaller keys yet:)

I know--gives me a headache. The iPad is my mom's; it's great for her as an e-reader but until the apps are killer apps it seems more like a toy to me (but I do expect that to happen, sooner or later).

I would have to say yes, that is correct. The audio, secondary audio, video, and various elemental streams that carry metadata are wrapped together in a multiple program MPEG transport stream, but after demuxing the unwanted stuff. And of course an MPEG2 stream is converted to MPEG4 before uplinking. But there is no "non-DD" audio to record, only the AC3 audio. If the source is 5.1 and the playback setup is 2.0, it downmixes the 5 full DD channels and the LFE channel to a full-spectrum L and R channel at the STB, whether that is used as the DD or analog audio output.

Your DVR can select any of 4 slots on a digital transponder, typically. They may encapsulate some channels together in, again, the DVB version of a MPTS during uplink. But just as your ATSC tuner can demultiplex out just the audio and video streams you want for a particular major or minor channel from a broadcast station, your DVR's DVB tuner can demultiplex out just the signals it wants as well.

But if a program has more than one pair of audio tracks associated with it, say for a second language, both sets will record, and playback settings at the DVR determine which audio is heard.

Edited by TomCat, 10 April 2010 - 06:56 PM.

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#13 OFFLINE   Carl Spock

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Posted 11 April 2010 - 06:32 AM

Something doesn't make sense here.

If you select PCM for the audio output, is that really DD 2.0?

I've always thought AC-3 and PCM audio were two separate things. Dolby Digital is compressed. PCM is not. PCM is just taking an analog audio stream and converting it to 1 and 0 without further processing the signal. AC-3 is much more than that.

In other words, are there two audio streams being recorded by the DVR, AC-3 and PCM?
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#14 OFFLINE   Beerstalker

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Posted 12 April 2010 - 09:05 AM

AC-3 is pretty much just Dolby Digital. It is the compressed digital audio included in the channels signal. The D* receiver then decodes it and sends it out as dolby digital (if DD is turned on), or decodes it further and sends it out as analog 2 channel PCM.

Kind of like how some Blu-Ray players can send out the Dolby TrueHD stream as a bitstream keeping it digital over HDMI, or decode it to multi-channel analog PCM and send that out over HDMI or multichannel analog outputs.

#15 OFFLINE   Stuart Sweet

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Posted 12 April 2010 - 09:08 AM

Something doesn't make sense here.

If you select PCM for the audio output, is that really DD 2.0?

I've always thought AC-3 and PCM audio were two separate things. Dolby Digital is compressed. PCM is not. PCM is just taking an analog audio stream and converting it to 1 and 0 without further processing the signal. AC-3 is much more than that.

In other words, are there two audio streams being recorded by the DVR, AC-3 and PCM?


Yes that is true to the best of my knowledge. There are two separate audio streams and only one is Dolby Digital.
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#16 OFFLINE   ricochet

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Posted 12 April 2010 - 09:21 AM

Something doesn't make sense here.

If you select PCM for the audio output, is that really DD 2.0?

I've always thought AC-3 and PCM audio were two separate things. Dolby Digital is compressed. PCM is not. PCM is just taking an analog audio stream and converting it to 1 and 0 without further processing the signal. AC-3 is much more than that.

In other words, are there two audio streams being recorded by the DVR, AC-3 and PCM?


The DVR will record all the audio tracks for a particular program but all of them are almost certainly compressed. Most likely AC-3 but not necessarily. TomCat stated earlier that HD always uses AC-3 but that isn't totally true. The ATSC standard for over the air broadcasts requires at least one AC-3 track but even there I'm pretty sure you can add other types if you want. Cable and satellite broadcasters are free to do pretty much whatever they want, but in practice I'm betting the HD tracks are always AC-3.

Back to the PCM question. DirecTV could include a full uncompressed PCM track (or DTS, TrueHD, or any other of the new codecs I can't keep track of) but for bandwidth reasons I'm sure they never do. When you select PCM output the DVR is decompressing the AC-3 track and feeding you that.

Don't get confused by Blu-ray. That is a different situation and the disc may (must?) contain uncompressed PCM tracks. The settings you use there may not be what you want to use on a DVR.

#17 OFFLINE   Carl Spock

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Posted 12 April 2010 - 03:39 PM

So Stuart believes an uncompressed PCM digital stream is being sent to us, along with the AC-3 one. ricochet makes a logical and compelling case for this not to be so. Any other experts out there with the goods?
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#18 OFFLINE   evan_s

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Posted 12 April 2010 - 04:03 PM

I agree with ricochet. I'm sure that the PCM output is just coming from the receiver decoding and downmixing the AC3 audio that is there. It would need to support decoding it for the analog outputs anyway. Including multiple redundant audio streams, which increases the bandwidth per channel, rather than using the decoding capability built into the chipset doesn't make any sense. Older SD mpeg2 channels might not use Dolby digital/ac3 but I'm pretty sure the only time there are multiple audio tracks is for multiple languages or for the mix channels where they are different audio feeds.

#19 OFFLINE   TomCat

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Posted 12 April 2010 - 08:35 PM

...TomCat stated earlier that HD always uses AC-3 but that isn't totally true. The ATSC standard for over the air broadcasts requires at least one AC-3 track but even there I'm pretty sure you can add other types if you want...

ATSC can use AC3, AAC, HE-AAC, or MP3. But there is likely not a single TV station or ATSC provider that uses anything other than AC3. There is no separate PCM or other audio sent. All other audio is derived locally at the STB from AC3 audio.

Cable and satellite broadcasters are free to do pretty much whatever they want, but in practice I'm betting the HD tracks are always AC-3...

Henry Ford would make you a car in any color you like, too, as long as it was black. DBS uses DVB, which not surprisingly has limitations well short of "whatever they want".

DVB-S/SH/SF can also use AC3, AAC, HE-AAC, or even MP3 (and not PCM), but DBS vendors likely follow the same economic model of not putting any redundant info into a signal which will be bandwidth limited, meaning they probably use AC3, since it it so very close to DD, which a lot of program suppliers use, and is probably compliant with even the older SD IRDs (sort of depends on when the newer codecs were incorporated into DVB, I guess).

And since MPEG4 typically uses HE-AAC, stuff that they generate locally may converted to that and sent as HE-AAC. Stuff that is already compressed to AC3 may also be converted to HE-AAC on the MPEG4 channels, but there could be enough of a quality hit during conversion at those low bit rates that they may very well just pass through the AC3 they receive. Also, it is not clear whether HE-AAC incorporates the DD downmix model needed for 5.1 audio and 2.0 downmix. IOW, using HE-AAC may be too cumbersome for and/or detrimental to sources that are DD 5.1 any part of the time. If so, AC3seems like a better choice, even for MPEG4.

If a particular channel sends AAC or something else to them on backhaul, DTV may take the easy path and send it as is, assuming the STB can decode it. If they send something proprietary, it is probably definitely converted to whatever the customer's local decoder can handle. If the older boxes are all HE-AAC compliant (which is doubtful) they probably convert to HE-AAC. If not, they probably convert to something (or pass through) all boxes can handle, which is most likely AC3.

AC3 has been around since at least 1992, back when DBS was still a gleam in DTV's eye, while AAC debuted in 1997 and HE-AAC became part of the MPEG4 standard in 2003. Since there may still be boxes out there pre-1997, that probably limits them to AC3 for SD channels, although they could use HE-AAC for some MPEG4 channels, except for those arriving by backhaul as AC3, which they probably pass through, especially since they centercut and down rez HD OTA channels for SD-only boxes. I guess they could have used MP3, but in 1994 AC3 was probably the smarter decision.

Whatever they choose, it is compressed. If there are analog or PCM outputs at the STB, those signals are derived locally from the AC3. Again, there is no separate PCM broadcast.

But, it would not surprise me to see some premium channels sent with DTS some day. Just not now, and not soon.

Edited by TomCat, 12 April 2010 - 09:07 PM.

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#20 OFFLINE   TMan

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Posted 03 December 2012 - 08:29 AM

I realize this is an old thread, but my question is essentially the same and duplicating in a new thread was unnecessary.

Based on the above, it looks like if I record something at the HR34, DD 5.1 is recorded whether DD 5.1 output is turned on or off at the HR34 or the box I'm playing it back on. If off, the HR34 or C31 will provide a PCM; if on, DD 5.1.

I assume this is still true two years later.
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#21 OFFLINE   jimmie57

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Posted 03 December 2012 - 09:11 AM

True.
A program is recorded as broadcast for that channel. The selections that can be made only alter or modify the output of the recording or viewing.

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