Simple Audio Fill tip!

Grazie wrote on 6/1/2006, 2:47 PM
I captured some footage of talking heads. Reporter's TX mic and XLR-ed Rode VidMic.

One of the channels was sounding weak and of course the subject was not where the audio was coming into the camera. The sound on one channel was sounding great, the other weak. Copied audio; pasted/placed/synched under orig. audio track in a new track; "Swapped" the channels and . . BINGO! Full sound that sounds fab! It was like that first time when I got to grips with Colour Correction and C.Curves. The audio just "popped" outta the screen at me . .

Vegas is great for making stuff like this soooo easy to do. I'm sure every other NLE does it, but this was done in less than 10 secs.

G

Comments

Former user wrote on 6/1/2006, 5:48 PM
Grazie,

If one channel is good and the other is bad you can also select to hear one channel only.

Right click on Audio track and go to CHANNEL and select LEFT or RIGHT only.

Dave T2
farss wrote on 6/1/2006, 7:40 PM
Better way,
copy the track, make one track Lchannel only and the other R channel only. Then you can deal with them like they were two mono tracks.

Bob.
Grazie wrote on 6/1/2006, 8:41 PM
Dave, Bob yup, that's good too! I just had this one event, amongst many. The resultant audio was great.

Grazie
farss wrote on 6/2/2006, 2:10 AM
Could I just suggest you check what you've done in mono. I think it'll be OK, sorry if you already know to check everything as a mono mixdown. There's a button in the mixer window to do this.

Bob.
Grazie wrote on 6/2/2006, 4:29 AM
Could I just suggest you check what you've done in mono. I think it'll be OK, sorry if you already know to check everything as a mono mixdown. There's a button in the mixer window to do this.

I know where the button is. I can hear the difference. Please explain further?

Grazie
farss wrote on 6/2/2006, 5:22 AM
Well obviously firstly mono is going to sound different to stereo, you loose the placement of sounds.

However it's not that hard to create prefectly OK sounding stereo that totally vanishes in mono! Just copy the a mono source to both channels and invert one channel, sounds OK still. Mix down to mono and 100% cancellation. That's the most dramatic example.

Consider two microphones, lets say two lapel mics, one on each person in a sit down chat. Both mics will pickup sound from both persons. Feed LH mic to LH channel, RH mic to RH channel. Sounds just fine in stereo. Various people around the network want to hear the interview so someone dubs to audio cassette, checks and it sounds fine. Producers go to play tapes but can barely hear anything. Problem is their players are mono. This did happen. Why?

Because of the time delay when the sound from the LH person reaches the two mics. That time delay translates in a phase shift between the LH and RH channels. Not a problem in stereo but add the LH and RH channels (i.e. mix to mono) and at some frequencies the two signals are in phase and add and at others they are out of phase and cancel. We have created a comb filter!

Some thing can happen with X-Y stereo microphones, that's why the BBC prefers MS mics for stereo. The LH and RH channels can be added to produce perfect mono. With X-Y pairs some cancellation can occur the mono mixdown. To minimise this the two mics in X-Y pairs need to be very close together.

As a general rule of thumb if the distance are greater than a factor of three you can ignore these problems. So say the two speakers with their lapel mics have the mics 400mm from their mouths. So long as they are sitting more then 1.2M apart all should be pretty much OK.

Now in your case you've got two mics picking up the same sounds, one a lapel very close to the sound source and one a lot further away. In each channel you've added those two signals together but they're delay in time, a delay determine by the distance between the mics. If it's a big distance then the on camera mic is probably picking up very little sound anyway and it's unlikely you'll have an issue, unless there's a PA involved! Of course reflections within a room would make it hard to know exactly what might happen, that's why regardless it's best to check in mono, even if the theory says it should be OK. Of course if you know it'll always be heard in stereo then I guess it matters not.

What should you listen for?

Dramatic shifts in the sound, like holes in the spectrum. If the bottom end drops out in mono you have a problem.
Phasing or flanging.

Bob.

DavidMcKnight wrote on 6/2/2006, 5:24 AM
Checking a stereo audio track in mono is a good practice to get into because you can have phase cancellations that can cause some - or all - of the audio to vanish. Not knowing the technical term, I'll put it this way - if the audio programs are of the same material and the wave on one is "up" while the wave on the other is "down" and you play them in mono, basically summing them - they cancel each other out and you have nothing.

This is also why when connecting speakers to your stereo that you pay attention to the red and black terminals and keep them the same for both. If one is wired differently, the two speakers are out of phase.


<edit> Yeah, what Bob said....
Former user wrote on 6/2/2006, 6:06 AM
Using my suggestion there is no problem with phase. The phase problem will come in if you use different audio sources of the same audio (such as two mics on the same person) or if you shift the sync of one of the mono tracks on the timeline slightly. ( by slightly I mean by samples).

Dave T2