Half-speed LP Playback

[Keith]

Does anyone have any experience with playing back LP's at 16 2/3 rpm and then speeding them up in DC? I understand there are advantages to be gained by reducing stylus velocity.....Are those gains worth the losses incurred by digitally juggling the RIAA equalization? Where does the band of "rumble" information go? Is it shifted further up into the audible region?

On a related topic....I've considered building a phono preamp stage with switchable/defeatable eq settings, from some plans I found on the internet. Does anyone have any opinions???

[greg_m]

Re: Half-speed LP Playback

There is a very small possibility that there is some rumble, which originated at the cutting lathe, actually recorded on the disc. If so, half-speed playback would shift it down an octave, and then the software up-shift would restore it to its original frequency. However, the likelihood of this type of rumble is very slim.

More likely, you have rumble originating with your playback turntable. This could be at two frequencies: one related to the motor's rotational speed, and the other related to the turntable's rotational speed. Depending on how your turntable accomplishes speed change, the motor speed (and rumble frequency) may be the same at either playback speed; or the motor itself may slow to half speed (for direct-drive or some types of servo turntables).

If your motor slows down, then its rumble will occur (and be recorded) one octave lower; the software up-shift will bring it back to where it would have been in the first place. But if your motor speed is constant and the turntable speed change was accomplished by a change of belts or pulleys, then the software up-shift will bring that motor rumble up an octave higher than it would have been.

The rumble from the turntable bearings (which is probably the predominant rumble) should be an octave lower than normal with half-speed playback, and then software will shift it back up to its "normal" frequency.

Incidentally, most rumble occurs in the vertical plane. This plane contains only the L-R or difference audio material. Since the rumble is usually quite low (let's assume below 50 Hz), and humans can't hear directionality below a few hundred Hz, there is a good trick for filtering rumble. Process the original two channels (L and R) into a sum (L+R) and difference (L-R) channel. Apply a high pass filter only to the L-R signal... probably fairly flat down to around 200 Hz, then sloping off to at least -20 dB by 50 Hz or so. This will remove most rumble quite effectively. Yet that L-R channel should contain little or no meaningful LF info, so you won't hurt the audio program material. Finally, re-matrix the sum and difference channels back to L and R stereo channels.

[Keith]

Re: Half-speed LP Playback

Thanks, Greg, for helping me with the rumble issue. Does anyone have any advice on my other questions??

[Rick]

Re: Half-speed LP Playback


The technique that Greg describes can also be accomplished by our Channel Blender. Rumble reduction on records is why we created that filter in the first place.
The technique is slightly different, but the result is the same.

On you preamp plans, what kinds of opinions do you want? It is definitly worth having such a device if you plan on doing much restoration work at all. Just having a RIAA preamp is too limiting. It is much better to NOT apply incorrect EQ in the first place than trying to unrangle it later.

Rick

[Craig Maier]

We offer a flat preamp on our website. It is model number DCP-47K-F. It works nicely with the Virtual Phono Preamp (VPP) in the Diamond Cut Software.