Paul R
pfm Member
A couple of weeks ago I came across this blog post and my interest was piqued.
In essence if a turntable plays a recording of a pure tone then any variations from a constant speed will manifest as a change in the frequency of the tone. If the tone is demodulated then what comes out is a moment by moment measurement of the instantaneous rotational speed of the platter.
It turned out that this book contained everything one needed to know to implement digital FM demodulation.
A few evenings later I had a couple of crude programs. One applies FM demodulation to a mono wav file, the other applies quite large FFTs to the result and writes a text file suitable for plotting.
Given that numbers are coming out how does one know they are sensible? I generated an FM modulated test signal that appeared to demodulate plausibly and then moved on to making some recordings of a test record.
As no record is completely concentric, nor completely flat, we would expect modulation at the rotation rate and at some multiples. For 33rpm this is 0.56Hz and multiples, for 45rpm 0.75Hz and multiples.
At present I only have an LP12/Lingo. It occurred that I have two ways of making it got at 45rpm, either with the Lingo driving the motor proportionally faster or using the original big pulley adaptor and having the motor go around at the standard rpm.
With the standard 50Hz drive the motor turns at 250rpm/4.17Hz. For 45rpm this has to rise to 337.5rpm/5.625Hz.
After making recordings in the two 45rpm states, processing the data and plotting the results produced,
which rather gratifyingly meets the expectations.
More later. And if anybody is prepared to make a recording of a tone it would be interesting to add.
Paul
In essence if a turntable plays a recording of a pure tone then any variations from a constant speed will manifest as a change in the frequency of the tone. If the tone is demodulated then what comes out is a moment by moment measurement of the instantaneous rotational speed of the platter.
It turned out that this book contained everything one needed to know to implement digital FM demodulation.
A few evenings later I had a couple of crude programs. One applies FM demodulation to a mono wav file, the other applies quite large FFTs to the result and writes a text file suitable for plotting.
Given that numbers are coming out how does one know they are sensible? I generated an FM modulated test signal that appeared to demodulate plausibly and then moved on to making some recordings of a test record.
As no record is completely concentric, nor completely flat, we would expect modulation at the rotation rate and at some multiples. For 33rpm this is 0.56Hz and multiples, for 45rpm 0.75Hz and multiples.
At present I only have an LP12/Lingo. It occurred that I have two ways of making it got at 45rpm, either with the Lingo driving the motor proportionally faster or using the original big pulley adaptor and having the motor go around at the standard rpm.
With the standard 50Hz drive the motor turns at 250rpm/4.17Hz. For 45rpm this has to rise to 337.5rpm/5.625Hz.
After making recordings in the two 45rpm states, processing the data and plotting the results produced,
which rather gratifyingly meets the expectations.
More later. And if anybody is prepared to make a recording of a tone it would be interesting to add.
Paul
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