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Paul D · Posts: 4

I have used Cool Edit (mix of filtering and noise reduction) to tidy up traffic rumble etc from a project which I have mastered to DVD.

The DVD master is being used to run off VHS copies, but there are several sequences of bird song and on the VHS copies each song is accompanied by deep, fluttery background noises.

There's nothing audible on the DVD master, but if I disable Hifi playback on the VHS machines (and just leave a mono soundtrack) the problem disappears, so it does seem to be something to do with the Hifi soundtrack.

A friend has suggested that Cool Edit has left behind artefacts which are inaudible to me, but which nevertheless interfere with Hifi playback. Having checked tape stock, leads, machines etc this is the only suggestion that fits.

Does anyone have any experience which would confirm that this is the problem?

And more importantly can anyone suggest a solution? I have to get some clean VHS copies out shortly, so this is an urgent problem and any assistance would be very welcome.

Craig Jackman · Location: Canada Posts: 909

Oh I doubt it.

If you playback the DVD and it sounds fine, the mono track on the VHS sounds fine, but the VHS hifi stereo track sounds funny, then the problem is 99 times out of 100 going to be in the VHS recorder, probably tape stretch or tape transport or phase error related to those 2.

Are you dubbing to VHS in real time or high speed? Are your connections analog?

If you don't hear it on the analog out of the DVD, it's not something CEP left behind.

Roger The Shrubber · Posts: 159

Back when VHS HIFI first came out, I was experimenting with dumping music to VHS for storage reasons. Some instances of music would create the effect you are talking about. From what I understand about the HIFI format, it uses a type of FM modulation. This is the culprit, not CEP leaving artifacts.

You might try experimenting with noise reduction. See if you can isolate an instance of the bird chirping, get a noise profile from that, then apply it to the whole soundtrack. I wouldn't try removing it completly (probably impossible anyway), but see if you can lower it to the point where it creates less fluttering to the point of being acceptable.

Paul D · Posts: 4

Thanks very much for the comments.

Craig I'm dubbing in real time to 8 VHS machines (via a distribution amplifier and yes I have tested that) and I can't see how all 8 could have developed a transport or other fault at the same time. The DVD output sounds fine to me, but if the sound artefacts were above the limits of human hearing I wouldn't hear them would I.

Roger the problem is the bird song is a very important part of the tape, it's in numerous places and can't be removed. If the cause is FM modulation how can I corredt it without damaging the integrity of the rest of the sound?

Thanks again for your comments - your input is really appreciated; I've got a bit of a hole to get out of here, so keep the suggestions coming.

Roger The Shrubber · Posts: 159

Paul, I did a Google search for vhs fm and other variants and it yielded little more then lists for vhs decks with the HIFI technology but I did stumble across one site with some info on the subject. http://www.geocities.com/SoHo/6413/AudiotekPress-VHS.html

I'm not sure if the problem can be corrected, just as mp3's have their inherant problems, so does fm modulation. Perhaps someone with more experience in fm could provide a solution.

Does the audio track have to be high fidelity? If not, you could record the tapes on a machine that doesn't have the FM capabilities or a pro dubbing machine that lets you choose the audio options and not record on the HIFI track.

Paul D · Posts: 4

Roger many thanks for that link. Your last paragraph about recording in mono is what I am doing (I've got three new mono machines working away now).

I'd like to think that this is a temporary solution as I would prefer to use the better quality of the Hifi soundtrack.

Many thanks again for your help.

MusicConductor · Location: USA Posts: 1524

Shame on your friend, Paul, for suspecting that Cool Edit would leave inaudible artifacts! It would never behave so covertly. Smile

I agree with the others that the problem is in the VHS HiFi system.

The noise you described is basically a hum with harmonics and unrelated impulses in sync with the picture field swaps (59.94Hz). In short, it's called head switching noise. It seems that all backwards-compatible technology has its compromises, and the cost of creating the VHS HiFi format is that the carrier frequency for the FM audio has to change slightly, back-and-forth with each field to keep it from interfering with the video carriers, or some such thing. Plus, the head has to switch from reading the end of one video track stripe to the next one, and it seems that the audio is related to this (although I believe that the audio is recorded with a separate head but is still on the rotating drum.). The problem is making the amplitude of the audio exactly correct at the moment of the switch.

Various manufacturers have varying performance in this area, though today in the world of $50 VCRs I have no idea how it goes -- it may be all from the same chip and assembly! But years ago I used a very good JVC machine that still has the lowest noise of this kind that I've ever used. A tape recorded and played back with it is closer to being devoid of this problem than anything else I've ever tried (unless mistracking). But tapes played back with it made on another machine could be a nightmare -- if I remember correctly, Panasonic was compatible and clean, and Sony-made tapes were full of loud static. There we had proof that not all HiFi audio was handled the same way. Interestingly enough, you could hear the hum pumping in on commercially made tapes, but rarely or never encountered bad static.

So I would recommend checking these tapes in several VCRs and seeing if the audio garbage remains. If it does, I'd try recording it on a different machine, at least as a test (sorry, I know you've got all that duping equipment).

Removing such noise in Cool Edit is very difficult; the hum parts of it are removable with Noise Reduction, but isolating an accurate sample is really not possible because HiFi audio is companded, so that the noise floor "pumps" with the audio program material. Worse, any static kinds of noises (sort of reminds me of spark-plug interference on an FM radio) are too irregular an impulse to use Noise Reduction to clear up, and are frequent and pretty long waveforms so that the Pop/Click tool would have quite a job on its hands. So it could be improved, but at what expense to the audio quality?

jester700 · Posts: 546

You hit it on the head, Bill.

The only solution I can think of would be to get the audio way up above the switching noise. Maybe some compression & level raising pre-VHS copy. That switching noise is obtrusive!

Paul D · Posts: 4

Many thanks for the detailed input which is really appreciated.

Bill you may well be right (I'll happily admit I'm no expert in these matters), but I'm puzzled that:-
(a) the problem is only apparent on bird song passages, you can never hear it in the numerous sections with only voiceover. If the problem was just head switching then there wouldn't be that correlation would there? That suggests to me that there is some other element as well. (b) I've done over a dozen other projects without this problem. I can't see any other significant new feature about this latest project except that I've used Cool Edit to tidy up the audio.

Thanks again for your help, if you can find the time to offer any further comments that would be great.

MusicConductor · Location: USA Posts: 1524

Thanks for the corroboration, Jester.

Paul, don't be puzzled. Head switching noise is absolutely the worst on isolated, higher frequency audio, and birdsong definitely falls into this category. This is for two reasons: precisely matching the amplitude at the switch points is more difficult with faster waveforms; and, there is no lower frequency content to mask the garbage. In fact, if you want to really hear how bad it is on any HiFi VCR, record a really high frequency sine wave and play it back. You might be amazed at how nasty it sounds. Voiceover has plenty of sound low enough in the spectrum to be much friendlier.

By the way, I believe the one analog FM-based format that doesn't have this problem is the analog stereo tracks of Hi-8 video. Not that it helps if your clients don't have a Hi-8 VCR!

York Audio · Posts: 80

Are you sure the problem is not just caused by playing back the VHS with the HiFi/Normal mix switch on?

SteveG · Location: United Kingdom Posts: 6695

I recall from a while back when I was involved in multiple dupes using Hi-Fi tracks, that this problem certainly occurred occasionally, and it certainly varies across machines. I think that one of the main reasons relates to how well the AM suppression is set up and operating in the head-switching period. There are certainly carrier level changes at head rotation (ie frame) rate, and these show up as AM on the FM output, and the extent of this to a certain degree is deviation-based, which is why it doesn't show up so much on signals with more LF content, and is absolutely awful with HF sinewaves, as has been observed.

You shouldn't get video interaction, though, because the audio and video are recorded using different azimuths on separate heads, the only breakthrough possible being at LF, and this should be rejected anyway.

But I think that York Audio may be right - you often get strange effects like this if you play your tapes with mixed audio set. And certainly my Hi-8 camcorder records pretty clean audio on the FM tracks, but I've never put it to any sort of a test, so I have no idea how well it copes with this sort of thing in terms of playback artefacts.