Cleaning up acoustic 78s & cylinders

[Dan McDonald]

Just a question -

After creating a wav file from an acoustic recording, should I apply "reverse RIAA phono) equalization settings before using the impulse filter? That doesn't make any logical sense to me, but it seems to result in a better overall sound.

Similarly, my turntable doesn't have a 78 speed. I've recorded 78s at 45 and then used the speed change to make it play at the correct speed. I get the best results running the "reverse RIAA Phono," then the impulse filter, then fixing the speed. It's just a little hard to adjust the impulse filter because the sound is so slow.

Anyway, do these sound like reasonable things to do with these media?

And one more thing I've found... I get a stereo signal from these early 78s and then subtract l-r, which gives almost complete noise (almost no real signal). Then I subtract that noise from the "true" signal. I've experimented with subtracting from the left, the right, and from the l+r combined. My sense is that it should be from l+r combined, but I don't know if that's technically what I should do or not. Any ideas? It seems to be a nice alternative to the continuous noise filter that pulls out a lot of scrapes, scratches, pops and high ends without hurting the signal.

Dan

[DrDave]

Re: Cleaning up acoustic 78s & cylinders

I would have thought you would have to fix the speed first before the Reverse RIAA could work correctly. Wouldn't recording at 45 change all the pitch? Or maybe there's something I don't understand.

[Dan McDonald]

Re: Cleaning up acoustic 78s & cylinders

Yes - it doesn't correct the eq correctly, but it seems to help the impulse filter work more efficiently - that is, I seem to get better results doing it this way than waiting til after.

I don't know why.

Dan

[Dan McDonald]

Re: Cleaning up acoustic 78s & cylinders

Now that I think about it, I think it is correct to apply the reverse riaa phono before the speed change, since it was applied at the slower speed during the recording process, right?

let's see... if you play something slower than it is, it makes it sound deeper; the riaa boosts the lows also; which makes it even deeper. Then when you apply reverse riaa it puts it back the way it should be at the slower speed, and when you speed it up, you get the 'correct' sound..right?

Dan

[This message has been edited by Dan McDonald (edited 09-22-2002).]

[greg_m]

Re: Cleaning up acoustic 78s & cylinders

AFAIK, the following should be true.

The reverse RIAA curve undoes the RIAA eq of your phono (playback) preamp. The specific frequencies of the reverse RIAA curve correspond to the specific frequencies of the preamps's RIAA curve. So you should apply reverse RIAA as the first step, before any speed change.

If you speed shift first, the preamp's RIAA frequencies get shifted up, to higher frequencies. Then the reverse RIAA curve won't correspond any more to the curve introduced by the preamp (since it's been shifted).

This is true whether your turntable is running at 33, 45, or 78 RPM. The preamp doesn't know the turntable's speed, so its curve is fixed at RIAA all the time. So if you're playing a 78 RPM (i.e. non-RIAA) disc on a 33 OR 45 RPM turntable, apply inverse RIAA immediately after you digitize the audio. Then apply whatever speed change you need to get the audio up to the correct speed (pitch).

[Dan McDonald]

Re: Cleaning up acoustic 78s & cylinders

Thanks, Greg. That is what I was thinking.

Dan