Audio all in Stereo
Submitted by Patrick Cody LPCTV on December 30, 2008 - 12:49pm
It appears that all of the audio of VMX content is normalized into stereo. We playback everything in mono, and in many cases, unless we tdemux / transcode the audio of VMX content, it plays back over our system distorted.
Does anyone else experience problems with converting stereo audio into a mono signal? How do you work around that?
I can change the transcode
I can change the transcode settings to use mono if that would be better for everyone. I should be able to get to that by early next week and then put up an update...
Is it possible to use dual
Is it possible to use dual channel mono for the transcode? This is what we need for playback. I can't say whether or not this is good for everyone, but judging from the lack of responses, I'm guessing it hasn't been too much of an issue for others.
I bet most people are
I bet most people are broadcasting dual mono, but there is another inherant problem with stereo to mono conversion. Phase cancellation. It's too in depth to describe here, but I will talk with Emily when she comes in to see if the VMX app can handle the phase shift to ensure no distortioin occurs. The other thing you can do is to delete one of the channels. You'll have to do this BEFORE you do the transcode. Open the m4v in QT and go to Window/Show Movie Properties. Then disable one of the audio tracks, then play the file to make sure you don't lose anything. If it still sounds good, select that audio track and click delete in the upper left corner.
Matt Goudey
Video Technology Coordinator
Vermont Community Access Media
208 Flynn Ave Suite 2G
Burlington, VT 05401
802.651.9692
Do we have a verdict on
Do we have a verdict on this? Should I make all VMX files mono? Just want to get this wrapped up before I head out of country for 2 months...
About to go into a staff
About to go into a staff meeting. I'll check back in on this when we're done. No verdict yet...
Matt Goudey
Video Technology Coordinator
Vermont Community Access Media
208 Flynn Ave Suite 2G
Burlington, VT 05401
802.651.9692
Patrick, I just re-read this
Patrick,
I just re-read this post and talked to Matt, and I don't think this is an issue with mono vs. stereo. First, do ALL VMX programs cause this distortion issue?
I think this might get back to audio normalization. When I discussed this with you way back when, I have some recollection of a discussion about your Comcast signal in general. Aren't most of your programs quieter than the VMX programs? Are you pumping up the signal on the Comcast end? I am wondering if when you transcode the VMX audio you are in fact lowering the audio level.
VMX audio normalization tries to get the audio at -12db for all the programs. I wonder if the final result is at a higher level than the rest of your programming and thus getting overmodulated when put over the air.
Can you refresh my memory on what level signal you are sending out? And do you know off the top of your head what your normal audio level of MPEG-2 is? If you play back one of your normal programs on a preview monitor and then play back a downloaded MPEG-2 VMX program on the preview monitor (that you haven't re-transcoded audio on), which one is louder?
Emily- This is not a
Emily-
This is not a problem with every VMX program. Only a select few..and it hasn't happened in a while. When it does happen, the problem is not over modulation - it is the distorted sound you hear when stereo audio is playing out of a mono output. The problem audio leves are not hot- they come in at -12dB..they just sound muffled and distorted. The on-air audio issue was solved last year when we first began experimenting with VMX. It was actually a MaestroVision problem, where the audio was set too low on the output - then when we played back the VMX programs it was blown out. But that is no longer the case. Thanks for exploring this, but I think for now we're all set. If and when we have the problem again we will just pull the program and post the issue here in the user forum.
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