TCV Signal Analysis
Speed
Because this sinusoid has a constant frequency, we can use this as a baseline for measuring differences in record speed introduced by touching the record or speeding up / slowing down the turntable motor.
Direction
Speed detection should work fairly well, but what if the record is not travelling forward? What if the motor is reversed or a hand pulls the record backward? Getting the speed might not be too hard, but determining the direction is a bit trickier. Notice in the picture above how the left and right channels are 90 degrees out of phase with each other. By comparing the values of the left vs. the right channels at a given time (and the sample immediately preceding), we can start to infer in which way the signal is travelling.
- If the values of the left channel are moving in the opposite direction as the values in the right channel (decreasing vs. increasing), then we know we are moving FORWARD.
- If the values of the left channel are moving in the same direction as the values in the right channel (decreasing vs. increasing), then we know we are moving BACKWARD.
Position
Decoding the position of the needle at any point on the record proves to be the most complicated aspect of a time code based system. It involves interpreting the waveform as a binary signal, and reading sequences of 'bits' to determine where in the usually 15 minute long time code signal we are at. The amplitude of the peaks of the sinusoid are used as markers, and come delivered in a sequence of lows and highs. How many bits to look for and the 'starting signal' sequence all vary amongst manufacturers.