The Secret Society of Lathe Trolls

tape · Joined: 25 Jan 2007 Posts: 93

I have somewhat strange problem, possibly in my brain:-)

Beacuse my cutter cut's with a lower speed than 33rpm, and I want to make 45 rpm on playback, I have to lower the pitch 9 semitones prior to cutting.

Now when I apply the reverse riia eq to this I guess the frequeny spectrum has moved?
I cant really figure out if it's best to eq before or after I pitch the audio down..

I know this is a strange question, but any comment will be welcomed...

cuttercollector · Joined: 11 Jun 2006 Posts: 307 Location: San Jose, CA

Ow! BRAIN HURTS ! : )
I gather you are doing all this in software thereby violating the "pure analog code" : ).
Nevertheless I shall help you anyway...
If you apply the normal reverse RIAA EQ curve before lowering the pitch it will all come out right because the curve does shift down with the frequency. You are shifting the pitch down by an amount that will cause it to come back to correct at 45 vs whatever your cutting speed is.
BTW some side effects of your "lower speed mastering" are, you get better high frequency response from your cutter because the actual top frequency it has to cut is lower so if it were true half speed (22.5 RPM) your cutter's 8Khz frequency limit would be extended to 16Khz. The down side is any turntable rumble is doubled in pitch and the low frequency cutoff if at say 40Hz is now 80Hz. This might cause you to have trouble cutting with enough bass level, so you might cut back on the lower part of the "bass cut" of the RIAA record curve back towards flat to prevent your recordings from sounding "thin".

tape · Joined: 25 Jan 2007 Posts: 93

Thanks cuttercollector - it makes sense now:-)

And you are right about the treble increase - now that is a fault I can live with!

Makes me think that this is something anybody could do - if the cutter motor is dependant on the frequency, it will only be a matter of getting hold on a voltage transformer with a lower frequency, and software to get the right pitch on playback....

Only one thing: how come 78rpm is always said to be better quality?
Faster speed must mean less treble....

cuttercollector · Joined: 11 Jun 2006 Posts: 307 Location: San Jose, CA

As I have stated before it's all about global system response capbilities vs level capable at those frequencies within the constraints of the amp and cutter ability. That's what makes analog audio so swell ! : )
So, your record curve at whatever speed you cut should by definition be whatever gives the flattest response with the lowest distortion at some reasonable cutting level for the system, using a good modern turntable/cartridge with a reasonably accurate RIAA playback curve as a reference point. I have suggested using a Real Time Analizer (RTA) used for setting up room EQ for live sound as an aid to doing this in other threads here. Keep adjusting your EQ while cutting pink noise till the result when played back on normal modern equipment is "flat" or as close as you can get it as viewed on the RTA.

cuttercollector · Joined: 11 Jun 2006 Posts: 307 Location: San Jose, CA

Posted 2nd reply while you were posting.
The facts are not contradictory.
Given any system the faster the media speed, generally the better the frequency response and signal to noise, so just like with 15ips analog tape being better than 3 3/4ips, so 78 is better than 33. There is just more media passing by to store information on. Kind of like digital sampling rate. The phenominon of getting better high frequency response out of your cutter is because of the fact that the actual material if it contained a frequency response of 20Hz-20Khz would at half speed be in the range of 10Hz-10Khz. So if your cutter has (and this is about it for those old crystal cutters) a response of 40Hz-8Khz it's response is more or less 80Hz-16Khz if you are mastering at half speed. This is because you are halving the frequency of the program being fed to the cutter then doubling it on playback.

cuttercollector · Joined: 11 Jun 2006 Posts: 307 Location: San Jose, CA

Last BTW, It is hard to change the line frequency. You can't do that with a transformer. And DON'T change the voltage to make it work at another speed. That does not work well at all with induction motors! What is needed is called a motor power amplifier which is a device which supplies 120V or whatever is needed with enough wattage to run the motor. It is essentially a big amp driven by an accurate oscilator at 50 or 60Hz whatever you want it to come out as. The catch is this amp has to be capable of delivering the voltage and wattage needed.
Most motors are pretty efficient between 50 and 60 Hz, but you could not probably apply this principle at 10Hz or 100Hz. It would not work as well.
The other approach, changing the drive ratio via a sleeve on the motor shaft to make the effective speed faster with the 60hz motor locked on 50hz is better. But please rebuild and restore the drive mechanisim to proper original condition before modifications.

tape · Joined: 25 Jan 2007 Posts: 93

just sharing my latest experiences while cutting at 'half speed'
I have managed to figure out that I have to pitch down the audio 9 semitones prior to cutting, in order to get the original pitch at playback at 45 rpm.

Now, in theory, this gives me a better HF response, and a poorer bass response (that can be somewhat corrected by adding more bass to the RIAA eq) - as Cuttercollector explained...

The problem with all this, at least on my machine (Bell, magnetic head)
is that the machine seems to resonate and distort at much lower cutting level, because of the pitch decrease - I guess this is just the way it is, and dont suspect that anything can be done about it...

In fact the sound is poorer now, than it was when I was cutting just roughly making an inverse RIIA on my 10 band graphic eq, and were cutting without changing pitch.....all just very 'midtone' now

hmm...