Rather than continue with the rather mediocre home-brew stereo MPX modulator I posted earlier, I’ve instead bought a used Rohde & Schwarz SML-01 with the B5 stereo modulation option.
Using a 1970s Technics SA-200L as the DUT, I connected the QA-401 left and right outputs into the SML-01 analogue inputs and the QA-401 input to measure the tape output of the DUT. Here are the resultant spectral and Bode plots:
Due to the worryingly high 19kHz pilot tone in the output signal, I set the measurement of THD to stop at 18kHz. This old tuner gives a respectable THD of ~0.1% when fudged that way.
It’s no wonder cassette decks need MPX filters!
The SML-01 has switchable pre-emphasis for 0, 50uS and 75uS allowing a region corrected response to be measured.
Ignoring the 19kHz pilot again, the frequency response of this DUT is just about ±1dB 20Hz-15kHz.
Once I’ve hooked up the SPDIF between PC and SML-01 I’ll see if the digital approach is any better.