clipping on analog capture

slacy wrote on 6/27/2005, 10:07 PM
Hey all,

I'm using Scenealzyer to capture some video that's being passed through a DV deck from a VHS deck. The audio is really hot, and I'm getting some annoying digital artifacts on the captured video. The peaks are tinged with what I would describe as a computerized "tinkling" sound.

How can I beat back these busted peaks prior to capture? Keep in mind that I'm a solo operator with limited equipment. My setup is pretty much a VX2000, Vegas, and an editing PC.

Any ideas?

Comments

Chienworks wrote on 6/27/2005, 10:30 PM
You need a pot of some sort to turn the volume down. A small mixer would do the job nicely. You could probably get a 2-channel DJ style unit for under $50. You could even buy $10 worth of parts from Radio Shack and build a passive volume control. Heck, i've seen some headphone extension cords with inline volume controls that might do the job if you're really desparate, but you'd probably spend more getting that and the necessary adapters than you would if you built something yourself.

Take a look at this: http://www.behringer.com/UB502/index.cfm?lang=ENG
Only $44.99 in the USA. There are a few on ebay right now with buy-it-now prices under $35. In addition to allowing you to mix two different line level audio sources together it also has a decent quality mic input as well.
B.Verlik wrote on 6/27/2005, 10:40 PM
You can set your normalization button to a lower db. (like maybe -3db) But if it's really coming in too hot, there may not be an easy fix. .
1st thought is, you could run the audio RCA cables from your playback VCR to your VX2000 and adjust the input volume at the camera. (probably will need manual to figure that one out) If that's not possible, you could try hooking the output RCA wires from your playback VCR and running them through a small audio mixer and adjust the volume at the mixer. Then output from mixer to your VX 2000.
If you don't have a mixer, you could run the RCA cables from the playback VCR through a Cassette recorder or another VCR, set them in record mode, but paused or not if you wish, then adjust the volume with the record levels and then run the output RCA from that unit to your VX 2000.
These are all the cheap options. That is, until somebody one-ups me.
slacy wrote on 6/27/2005, 10:59 PM
Thanks, guys. I guess I'll have to find something to insert into the analog chain. I'm using a cheap consumer VCR so there is no volume control to use.

Ah well. Looks like a trip to Radio Shack is in my future.
PeterWright wrote on 6/27/2005, 11:10 PM
The only way to avoid that purchase is to capture the audio through your sound card as a separate operation, then use the waveforms to synch with the original.

This way you can control the record level with the Windows controller.
B.Verlik wrote on 6/28/2005, 12:53 AM
That's actually a good idea I forgot about.
farss wrote on 6/28/2005, 5:03 AM
Even that may or may not work, many soundcards don't provide a line input level control.
I've dealt with this problem, a dual gang 50K log pot in a small metal box and 4 RCA sockets isn't that hard to throw together.
I've also added a pair of 10dB XLR pads to my kit as well to cope with that silly 'consummer' V 'Pro' line level problemo.
Bob.
TheHappyFriar wrote on 6/28/2005, 6:09 AM
using the sound recording into the sound card directly from the VCR won't work (well, it might, but odds are it won't). the camera converts everything to digital (we all knows that. :) ) & synces it nice & good. the VCR will playback at slightly varing speeds. If the tape is a few minutes long this won't be a problem, but trust me... syncing consumer analog anything to digital anything that isn't record at the same time will be hell. Even syncing consumer analog to consumer analog is a bitch... there's so many slight variations in the speed it'll drive you crazy. Look at ME. I'm rambeling on & on about it. It obiviously has effected me in a negative way. :)

there is a couple ways to make this work if it's long (30 minutes +).

a) record the entire thing analog (video & audio) AT ONCE.
b) record the video digital & the audio analog direct from the VCR. However, I'm pretty sure you'll need another digital destination for the audio besides your computer. Maybe not though. If you record your audio to a different HD then the video, you should be ok.
c) do the volume control in the line thing.

but if the audio is hot o nthe tape there's nothing you can do. Also, if it's not hot on the tape but for some reason it's the electronics on the VCR( dirty or soemthing) then it will clip no matter what.

If you have trouble it might not be a bad idea to check out the local TV station & see if they can put it on a DV tape for you. Odds are they got a broadcast VHS/SVHS player.
John_Cline wrote on 6/28/2005, 10:28 AM
The cheapest and most effective route is the method described by farss; a small metal box, two potentiometers and some RCA jacks. Here is a link to a schematic I posted years ago for building a device to attenuate the audio input to the old Miro DC30 video capture card. It shows one channel, you'll need two. I suggest two pots instead of a dual-ganged pot so you can adjust for channel imbalance.

Audio Attenuator Schematic

John
PeterWright wrote on 6/28/2005, 5:51 PM
You have a good memory John - do you still have your dc30?
Mine's over there on the shelf, complete with break out box.
slacy wrote on 6/28/2005, 5:52 PM
Well now I'm really stuck. Due to time constraints, I decided just to grab a cheap mixer from Radio Shack to complete this job. So I plug everything in, knock down the levels, capture and ... get the same result.

At this point I decided to check the tape itself (which I should've done in the first place, but I was so convinced that this was an A>D problem). Sure enough, there are those digital-sounding squeaks are various points in the audio, though not as frequent or as prominent as in the captured version.

Oddly enough, many of the squeaks don't even correspond with the peaks in the waveform. They seem to match the points of emphasis from the speakers, but not the highest waveforms themselves.

Anyone have any idea how to best deemphasize these digital squeaks? I'm not looking for a miracle here, just the best tactic for making the best of the situation. Because, as I like to say, the pee is already in the pool here.
TheHappyFriar wrote on 6/28/2005, 7:33 PM
they could show up in the waveform as a sample or two.

the only thing you can do is add an audio envelope & manually adjust hte volume. Or, delete by hand (1-5 samples doesn't make much of a difference). Or.. use Noise Reduction
slacy wrote on 6/28/2005, 7:39 PM
Thanks for the ideas. The squeaks are probably two numerous to surgically remove by hand. As for noise reduction, that's problematic for two reasons: 1) I can't sample the noise without sampling the voice, and 2) NR, in my experience, will tend to introduce those digital squeaks all on its own. That said, I'll give it a shot and see how it sounds.
slacy wrote on 6/28/2005, 7:49 PM
Since I'm not sure if "digital squeak" is the best way to describe this sound, I figured I should upload a sample and let those interested characterize it for themselves.

There are three squeaks in this clip. One right at the beginning, two more closer to the end.

Sample
PeterWright wrote on 6/28/2005, 8:18 PM
Warning - this is a "seat of the pants" approach - hopefully audio experts may have more clinical methods ....

Had a quick look and there appear to be audio "anomalies" - irregular shapes in the wave form - at the click points - for test purposes I removed one in Cool Edit - zoomed in to the "click", selected bad area - set zero crossings and deleted. This did the trick, but of course makes the audio fractionally shorter, so I noted the amount I deleted (.002 sec) and copied/pasted .002 worth of quiet just before that word, to keep in synch. HTH

Don't know what software you have, I'm sure Sound Forge has same or better tools -( I have SF but know CoolEdit better!).

Edit : You can do something similar with Vegas, but it doesn't have an auto-zero crossings tool, so you'd have to do this visually after zooming in.
PeterWright wrote on 6/28/2005, 9:04 PM
Just checked - what I was removing were "clicks", then I heard what I think you are referring to as "tinkles" - those I heard were at the capitalised points >
" ... are NUtritionally ...." and "... for HEALTH and enjoyment ..."

Anyway - I managed to get rid of these by applying a FFT Low Pass (4000Hz) filter. If used, it should be very sparingly so it doesn't make the voice sound different

This may equate to something you can do in Vegas, but I don't know the equivalent - anyone?

- must get back to work!
slacy wrote on 6/28/2005, 9:57 PM
Yes, the tinkles are what I'm trying to get rid of. I'll have to find the Vegas equivalent to this low-pass you speak of. If anyone knows, please clue me in.
slacy wrote on 6/28/2005, 9:57 PM
btw, thanks, peter.
PeterWright wrote on 6/28/2005, 10:13 PM
Try this:

Resonant Filter - Low Pass, Frequency 4,057

Either apply to whole track, or if just to tinkles, split these and move them down to their own track.

I also was able to remove the clicks, such as at "ARE nutritionally perfect" by zooming in, splitting and deleting (Match wave slope carefully), then replacing with equivalent space in the gap before the word.

Good luck.
slacy wrote on 6/28/2005, 10:44 PM
Peter, you are a GOD.

That worked remarkably well. I only pick up a hint of the tinkle in just a few places. The rest seem to be gone completely.

How did you hone in on 4,057?

Thanks a ton. You've made one guy's night.

scott
PeterWright wrote on 6/28/2005, 11:00 PM
Great to hear that it helped!

4000 was the setting which worked in Cool Edit, and 4057 was as close to 4000 as I could get with a quick try at the Vegas slider!!
farss wrote on 6/29/2005, 3:25 AM
I haven't tried looking at the sample in SF but they sounds awefully like digital dropouts, hear them from time to time coming off MiniDV tapes.

[edit]
Opened the sample in SF, definately not clipping but digital dropouts, the area where they're happening shows odd spikes at around 8,10 and 12 KHz. Use stacked low pass filters at say 6KHz with max rolloff and max attenuation should get rid of them. You may need to render out and apply filter more than once to get a sharp cutout. Before doing that drop the level back 6dB otherwise the knee in the filters gain will cause clipping.

If this was captured from DV tape best solution would be firstly to try recapturing. If several goes don't cure the problem and you have the gear feed the analogue audio outputs from the deck into your sound card and try recording that. I know this sounds bizzare but sometimes it works. Needless to say you'll have to resync the new audio by hand.
Bob.
slacy wrote on 6/29/2005, 9:22 AM
Just out of curiosity, how does a low pass filter work? Is it merely a different approach to selectively scooping out parts of the EQ spectrum? Or is there more to it than that.

Yes, I'm a big-time novice. ;)
TheHappyFriar wrote on 6/29/2005, 9:37 AM
I noticed (by turning up my speakers a little) that the tinny "squeek" that appeared was around the 7.5khz zone. I use the graphic EQ to eliminate it. I first lowered everything to 0db then moved them up one at a time until I found the freq to get rid of.

I have a headache today, so it could be just me, but somebody see if this sample sounds better then the one slacy put up:
http://www.sterlingshield.net/home/steve/sample_X_7.2khz.wav