Audio mislabeling?

virbing

Member
Hi.

I have been using an output profile with automatic formatting of the audio channels. I am expecting VideoRedo to read the format and channels for the input file and output the same audio format and number of channels without me having to read the file manually and choose the proper settings. Unfortunately, I am seeing inconsistent results.

Today I was trying to edit and re-encode content with 2 ch of AC-3. When I look at the source file (mpg) with Mediainfo, the file shows both audio programs as AC-3 2ch, with stream 1 at a bit rate of 384 kbps and stream 2 at a bit rate of 96 kbps. When I look at the resulting mp4 file with Mediainfo, it shows stream 1 as AC-3 6ch and stream 2 as AC3 2ch audio.

I am attaching a zip file with the log, pics of the profile and video program info from VRD, and the source and output info from MediaInfo. Looking through the log, it appears that VRD does interpret the incoming audio streams correctly so I am wondering why the output is showing audio stream 1 as 6ch. Does this have anything to do with the bit rates? I have done the same analysis with another PBS program that shows the source file as being 192 kbps and that one correctly output a file showing as AC-3 2ch in Mediainfo. It does seem strange to me that a 2ch signal would be taking up 384 kbps (full bandwidth of AC-3?) so I am also wondering if Mediainfo may be misreading the channel configuration.
 

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Dan203

Senior Developer
Staff member
I'm betting that your file actually contains a mix of 6ch and 2ch. Open the file in VideoReDo and display the thumbnails. Make sure it's set to single frame. Look at the audio visualization below the video frames. Those with yellow are 6ch (i.e. 5.1) and the ones with blue are 2ch. AC3 can have both mixed together. When we save we just look at the first audio frame that hits the muxer and set the MP4 header to whatever that is. In your case you likely made a cut that put the start of the file on a 6ch frame which is why that stream is listed that way in MediaInfo.
 

virbing

Member
Dan,
Thanks so much! Knowing that the audio streams are color-coded is exactly the information that I was looking for. It looks like HBO might be using 2ch in the preview part of their stream before going into program.

So if I put the preset into auto, will the AC-3 automatically switch between 2ch and 6 ch in the same program? Or do I have to go down to the 6ch (yellow) version and cut there in order to have the content play in 6ch?
 

cp2

Member
Some of the tv channels I record will have the actual programme as 6 channel but switch to 2 channel for the breaks. In addition there can be a delay to the 6 channel engaging so the first few frames can be indicating 2 channel sound. As I understand it VRD will decide the dominant channel count so that the output will have only one channel count. So, on occasion my 6 channel programme will be output in 2 channel. However, you can override the auto calculation and “force” 6 channel in the advanced settings.
 

Dan203

Senior Developer
Staff member
Dan,
Thanks so much! Knowing that the audio streams are color-coded is exactly the information that I was looking for. It looks like HBO might be using 2ch in the preview part of their stream before going into program.

So if I put the preset into auto, will the AC-3 automatically switch between 2ch and 6 ch in the same program? Or do I have to go down to the 6ch (yellow) version and cut there in order to have the content play in 6ch?
Auto passes through the audio as-is, so it will not change the channels. So it'll switch back and forth just like the original does. If you want to force it all to 2ch you have to change the codec to AC3, then changes the channels to stereo. That will force all audio to be recoded to 2ch.
 

Dan203

Senior Developer
Staff member
Some of the tv channels I record will have the actual programme as 6 channel but switch to 2 channel for the breaks. In addition there can be a delay to the 6 channel engaging so the first few frames can be indicating 2 channel sound. As I understand it VRD will decide the dominant channel count so that the output will have only one channel count. So, on occasion my 6 channel programme will be output in 2 channel. However, you can override the auto calculation and “force” 6 channel in the advanced settings.
This can be a good parameter for ad scan. In v6 we have the ability to scan the audio as well as the video. You can set it to mark a commercial when it detects a transition from 6ch to 2ch. Probably one of the most accurate indicators of a commercial you can use if your recordings regularly use that technique.
 

virbing

Member
Auto passes through the audio as-is, so it will not change the channels. So it'll switch back and forth just like the original does. If you want to force it all to 2ch you have to change the codec to AC3, then changes the channels to stereo. That will force all audio to be recoded to 2ch.
Dan,

Thanks again. This certainly does explain the question about bitrates I was asking about. Doing the math, it appears 384kbps leaves room for 6ch audio and allows for switching between 2ch & 6ch, 96kbps would be 2ch and 192kbps would be discrete 4ch - stereo + center + mono surround.
 

cp2

Member
I avoid AdScan but maybe i shouldn't!
I now have to correct my earlier post after studying an output file in VRD. I could see that VRD was switching between 2 channel and 6 channel as Dan203 stated. However, MediaInfo reports the file as 2 channel presumably from the first few frames. As I tend to use MediaInfo as a quick and dirty indicator of a file's spec (perhaps not a good idea) I'm inclined to continue forcing 5.1 in order to continue q&d.
 

Dan203

Senior Developer
Staff member
Both AC3 and AAC can contain a mix of 2ch and 6ch. Both MediaInfo and VRD program info will report the attributes of the first few frames so it may not be an accurate representation of the file. In fact it's possible for a file to contain just a second of 2ch audio at the very start and for the rest to be 5.1 and both programs would report the file as 2ch. The opposite could be true as well.
 

virbing

Member
I avoid AdScan but maybe i shouldn't!
I now have to correct my earlier post after studying an output file in VRD. I could see that VRD was switching between 2 channel and 6 channel as Dan203 stated. However, MediaInfo reports the file as 2 channel presumably from the first few frames. As I tend to use MediaInfo as a quick and dirty indicator of a file's spec (perhaps not a good idea) I'm inclined to continue forcing 5.1 in order to continue q&d.
This was exactly the rabbit hole I fell into. It started with the realization that output profiles for Apple formats were all set to 2ch AAC. Making programs to play back on an Apple TV (as well as for the virtual Dolby surround coming to the airpods) I made the decision to start encoding all of my content in 6 ch formats. But then to save space, I decided to check on every piece of content as it passed through so that I could match the source audio format for my recordings. After going around in circles, it now appears I should just trust the VRD auto audio settings and edit the Apple profiles with automatic audio settings.
 

Dan203

Senior Developer
Staff member
AFAIK portable Apple devices still don't support 6ch audio, or even AC3. Only the Apple TV supports that. Although I haven't kept a super close eye on it so it's possible that recent devices or OS upgrades changed that.

What I wold recommend if you want to make a profile that works both in full surround for Apple TV and still plays on a portable device is to duplicate the stream. In the profile options there is a little radio button over the Audio options that allows you to change to advanced mode. In advanced mode you can duplicate streams. So what I would do is set one take the primary stream and pass it through as-is using Auto. This should work on the Apple TV. Then set the second one to duplicate the primary stream and recode it to 2ch AAC. That will ensure compatibility with portable devices. The device you're playing on should automatically choose the stream that best suits it's needs.
 

virbing

Member
Dan,
We discussed this earlier when I started this long journey to multichannel audio on iOS devices. Apple portable devices have supported AC-3 since iOS 9.3. And then in iOS 14 this is coming, https://www.techradar.com/news/airpods-pro-to-get-dolby-atmos-spatial-audio which will allow for virtual surrounds on an iOS device with the Airpod Pros.

Apple devices have long supported AAC 2ch as its native music format, but as I discovered earlier, only the Apple TV and desktop apps currently support AAC 6ch. Portable devices supported AAC 6ch up until iOS12 but removed it, possibly because they knew of the upcoming Dolby Atmos support.
 

Dan203

Senior Developer
Staff member
I'll take your word for it. Transferring a video from a PC to an iOS device is such a PITA I haven’t bothered in years.
 
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