I have been messing around with my MD recorder again this morning and I
think I may have figured out what is happening.
The results I posted previously show a frequency dependant loss in volume,
which increases as the frequency increases.
If the signals from the two input channels were out of phase with each
other when they were combined, some interference would occur. The loss
in amplitude depends on the phase difference. For example, if the phase
difference is pi radians, or 180 degrees, the two signals cancel out.
For general phase differences the amplitude is always less than the
original.
Hence a volume loss will occur if the two identical input channels are out
of phase when added together.
As the amount of volume loss increases with increasing frequency, it is
clear that the phase difference is increasing with increasing frequency.
As my results cover 1-20kHz, the phase difference between the two input
channels is increasing over this range. As the volume is never reduced to
zero, the phase difference must be less than pi radians for all input
frequencies in this range, and it also must be increasing from 1kHz to
20kHz.
Returning to my previous idea of the channels being out of sync. with each
other, this type of phase difference could be caused by a synchronisation
problem between the two channels on combination.
If the two channels were out of sync. by one sample then it turns out that
for zero volume mono output, the frequency of the input wave would be f/2
where f is the sampling frequency of the input signal (I will post a proof
of this later). f was 44100Hz for my experiment, giving an input
frequency for total volume loss of 22.05kHz.
This fits quite well with my data, because the volume was very small at an
input frequency of 20kHz.
Because of the good fit with the experiment I decided to retry my initial
experiment with a data source where the channels were out of sync. by one
sample. To do this I wrote a small C program (email me if you want a
copy) to read in a wave file and write it out with the left channel one
sample behind the right channel (a guess: I had a 50% chance of getting it
right), and redid my experiment at a frequency of 16kHz. This time the
volume loss was greater than before. So, I tried it the other way round:
the right channel one sample behind the left channel. Voila! there was no
volume loss!
I retested all the frequencies I used in my experiment from 10-20kHz and
none suffered any volume loss with this adjustment.
So, to conclude, Sony's monauralizing algorithm has the channels out of
sync. by one sample, with the left channel one sample in front of the
right channel. I don't think this is occuring in the SPDIF data stream
because the synchronisation information embedded wouldn't allow this.
Instead I think there is a bug in Sony's DSP software.
I will post a more mathematical proof of this at a later date.
Jonathan
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