Brand new Teac turntable, getting asymmetric wave with each recording

[xeryus]

Hi everyone. I’m new to the forum, but not really new to audacity. I’ve used it for years for cleaning up audio tracks. Just got a new turntable which arrived yesterday, and started recording a few old vinyls for archiving. Actually, testing it out. I’ve never done this before.

The turntable is Teac TN-300. Has a built-in pre-amp. Also has a usb line (which I’ve used in this instance). I haven’t used the phono leads, they’re not suitable as they don’t go to a single jack. So, it’s turntable ->laptop via usb. Nothing in between.

The recordings I’ve done all seem ok except that they’re top heavy asymmetric. This has been with different records - so it’s not just one record that could be the problem. Here is a screen shot of the whole of one side of a 12" dance record.

Yesterday, I tried a few times with the normalize and remove dc offset, but the asymmetry doesn’t go away. The above screenshot is straight after today’s recording with no attempt to fix anything.

So, I’m just wondering what’s wrong and what I can try. I’ve seen plenty of symmetrical waves on my downloads, I’m not used to seeing this.

Thanks in advance.

[xeryus]

I’ve just done a bit more investigating.

I’ve done another recording this time, as audacity seems to advise, with the peaking not going much above -6dB, by lowering the recording level. I always thought you recorded as high a possible before clipping. Anyway, the result was this (screenshot 1, below).

It is still asymmetric after amplifying.
When going to spectral view, there is noise below 20Hz (screenshot 2, below).

Thinking that could be the cause, I did a cut at 20 with the spectral shelf, then did a dc offset/normalise to -0.3dB (each channel individually), and the result was as you can see in the last screen shot.

Am I doing the right thing here? It looks better with the low cut. Does everyone here who archives their vinyl in this way have to do this? Thanks in advance.

[DVDdoug]

This is probably normal. It shouldn’t hurt the sound quality at all. Sound waves are not always perfectly symmetrical. It could be the phono cartridge but I wouldn’t worry about it. The spikes (record clicks?) during the quiet parts are in opposite directions which is a clue that particular asymmetry is coming from the record.

…I did a cut at 20 with the spectral shelf, then did a dc offset/normalise to -0.3dB (each channel individually), and the result was as you can see in the last screen shot.

A low-pass filter (~20Hz so you don’t loose any bass) will often even-out the waveform no matter what the cause (without changing the sound). You don’t want a shelf, you want to reduce the subsonic noise/information as much as possible.

Yesterday, I tried a few times with the normalize and remove dc offset, but the asymmetry doesn’t go away. The above screenshot is straight after today’s recording with no attempt to fix anything.

You don’t have a DC offset. A true DC offset will show-up during silence. And you’ll hear a “click” at the beginning and end where the offset suddenly kicks-in and kicks-out.

Some DC offset correction algorithms shift the whole waveform up or down and this can actually introduce an offset.

I’ve done another recording this time, as audacity seems to advise, with the peaking not going much above -6dB, by lowering the recording level. I always thought you recorded as high a possible before clipping. Anyway, the result was this (screenshot 1, below).

About that…

The idea is to leave some headroom to prevent [u]clipping[/u] with unexpected peaks. Nothing bad happens if you get close to 0dB but you’ll get hard clipping of the ADC (analog-to-digital converter) if you “try” to go over. So… If you have 6dB of headroom after recording you didn’t really need the headroom!

Also, digital has a very wide dynamic range (at 16-bits or better) so nothing bad happens at low levels until you get down below about -20dB. Pros typically record around -18dB (at 24-bits). You can simply amplify digitally after recording. (Pros are usually recording “live” so there is a greater chance of unexpected peaks.)

I’m pretty sure your turntable doesn’t have an analog gain/level control so you can’t control the analog signal to the ADC so there’s no benefit to lowering the recording level digitally.

Here is a screen shot of the whole of one side of a 12" dance record.

At first glance I thought you had clipping. It would be analog clipping of the turntables preamp since it’s happening below 0dB, but since the peaks are not “perfectly flat”, I’d say it’s just a highly-compressed and limited [u]loudness war[/u] production. (The original master may have been intentionally clipped… The vinyl cutting & playback process changes the wave shape and it can “hide” clipping.)

[xeryus]

Thank you for your input there. Very helpful.

I thought I was on the right lines with recording high as poss without clipping. I give it a trial run through for a few moments, usually, then go for record. You’ve made me more confident there. Although what you told me about nothing bad happening at lower levels with the bigger dynamic range helps a lot. I actually feared getting too close to noise levels by being 6dB lower than my usual recording level.

The record clicks, yes. I’ll look out for those as I do more records. I did wonder why they seemed to disappear on the next recording (only difference being drop in recording level). I expected them to be less, but not to vanish. Maybe it’s just me.

A low-pass filter (~20Hz so you don’t loose any bass) will often even-out the waveform no matter what the cause (without changing the sound). You don’t want a shelf, you want to reduce the subsonic noise/information as much as possible.

Yes, I have read a lot over the time I’ve been interested in cleaning up audio. Thank you for backing that up. I have a question, since you say don’t use a shelf. When in spectral, there are three options for edits - a notch, a parametric, and a shelf. Should I use the parametric then, when doing this below 20Hz? The last thing I expect you to say would be to use a notch filter, but suddenly I feel I’m in the presence of someone who knows better…

Yep, the TT doesn’t have an analogue gain. The very first time I tried a recording, I did the usual monitor the peaks for a few moments, surprised that they were so close to 0dB, then just let it record. I was prepared for a clip at any time but the recording went just fine. Only the asymmetry stuck out, once it had recorded - so that went in the recycle bin. So, yes I thought, when I started looking at the mixer, why drop the level. Even a few clicks didn’t clip. As I say, it’s all new to me. Now I realise, no benefit to dropping the level unless of course I get some clip, for whatever reason.

Very interesting, your last part about clipping. I realise the waves don’t look like they have any dynamism. Of course I have read about loudness war stuff. I suppose it may have been about in 92, when this dance track was produced. On the other hand, it is a full on funky house record - that will have some bearing on it, won’t it I suppose.

So, thank you very much - a big help. And I’m glad I was able to get the asymmetry to diminish while waiting for an answer. Not sure it’s all completely gone, but it looks much much better. I thought there was something wrong with my new TT and was feeling disappointed. I would like your opinion on what to use in spectral mode other than the shelf - which you advised not to use. I will take your advice.