October 24, 2019

Is the GS Sachdev LP which I just posted mono or stereo? An investigation with pictures

I apologize in advance for making this reply to "ljf" as a post rather than a reply inside the comments section. There are two reasons: first, I have been having trouble getting my comment replies to "publish" on the page and I haven't found a solution to that yet. Secondly, I would like to reply with pictures and as far I know Blogger won't allow me to include photos and screen shots in the comment section.

As a recap, this is the original comment:


Thank you Richard for this download. I am intrigued by one thing and that is the rip is in mono. I have a copy of the CD and that is definitely in stereo, although the two tracks are very similar and both flute and tabla are close to centre stage. I'm sure that a US label of that period would not be putting out mono LP's, but you never know?



To destroy the suspense, I believe this record is a Stereo record with predominantly mono characteristics (in other words, the two channels share an awful lot of information).

First a couple of technical observations:

1) There is a difference between "true" stereo and "true" mono records and this will be seen upon visual inspection of the grooves. For stereo recordings, the stylus will move both "back and forth" across the groove as well as "up and down" in space. For mono recordings from before 1968-70, only information in the horizontal plane ("back and forth") is embedded in the groove. Using a "true" mono cartridge which is not compliant in the vertical plane will result in much less surface noise being heard. I like the Audio-technica AT33MONO cartridge for this job. It balances performance with cost (some mono cartridges can be priced over 2,000 USD).

2) Any "mono" LP cut and pressed after 1970 outside of India will have been cut with a stereo cutting head and can be playable with a stereo cartridge. So let's say I buy a recent LP of Benny Goodman's 1938 Carnegie Hall performance. The signal will be mono, but the lathe will have had a stereo cutting head and there will be duplicate information in both channels. Some people call this a "2-channel mono" record. I don't know enough about LP production in India in the late 1960s and the 1970s to say when true mono records were last made there, but usually because of import taxes the ability of Indian manufacturers to use the most up to date technology was limited. In any event this is a US recording and pressing.

3) Most of the time when you go to a concert and there is a PA system, the live mix will be mono. Sometimes it is stereo but generally most of the time it will be the same signal coming out of the speakers on both sides of the stage. This allows people sitting somewhere other than dead center to hear the same mix as someone else in the auditorium. It is also a lot easier to accomplish a mono mix. I am not a professional sound engineer but I have been called on to produce live sound for concerts several dozen times in my life (every time against my better judgement) and I have generally done a mono mix, even for small concert spaces. The very last time I did a live mix, I had the tabla "panned" 10% to the left and the sitar panned to the right 10% and several people noticed and commented about it to me afterward.

4) This is a live recording and was almost certainly recorded as a "soundboard" mix with or without additional channels from microphones on stage or in the audience.

5) Often on a stereo LP, the audio information below a certain frequency will be "summed to mono." I have read various articles suggesting this frequency could be anywhere between under 60-150Hz. The theoretical justification for this (I believe questionable) practice is essentially because humans have a hard time mentally locating the direction of lower frequency sounds compared to higher frequencies. Also some people claim that this summing to mono makes it easier for inexpensive cartridges to track the groove. Some cutting engineers will agree to not "sum the bass to mono" but usually will specify that they won't guarantee that the LP will play correctly. 

6) This label only released recordings by GS Sachdev so I think we can realistically refer to these Chandi Productions LPs and cassettes as self-released, probably with a very small production budget. It certainly could have been a mono master recording but there is no specific reason to think it was.


So when I read the comment by "ljf" I thought that I could investigate the files using the Audacity software which I love.

First I opened up the 24bit 96kHz file and looked at the wave forms. There were very similar but not identical waveforms in both channels (look at the first few minutes which are being shaded here -- definitely some different waveforms there:



Then I split the files into two separate channels and "inverted" one channel:


So now the left channel is the mirror image of its former self. If you do this with a two-channel recording, first changing the "left" and "right" channels to be both "mono" channels, and then sum back to one channel of sound by "exporting" the data to a mono output...


...you will obtain a one-channel file of just the difference between the two channels (because the common audio material of each channel will cancel itself out... when the wave in one channel is going up, the other channel will have a wave going down). With a completely mono two-channel recording, the result would be silence.

I eventually wound up with this awful-sounding file:


As should be apparent, there is indeed some material not common to both channels, and this amount is very small (note the small amplitude of the waveform). Interestingly, the amount of "stereo" content varies -- sometimes it is very minimal and occasionally it is quite a bit.

So in summary, I believe this is a relatively-inexpensively-produced recording derived from concert material, which probably was originally a mostly mono mix with stereo features.

Lastly, any blog readers who use the software "ClickRepair" should be on the lookout for the "Convert stereo to mono" setting... look at this screenshot and find the box in the very right lower corner -- this should not be checked unless you know you are dealing with a mono recording. When I first read the comment I thought that perhaps this is what had happened, but i no longer believe that.




Thanks to everyone for reading my blog and carefully listening to the LPs and cassettes posted here. More coming later this week. Think "violin" and "veena" and "cassette."


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