Mono on CD - Channels Different?

2009-12-10 14:57:37

Hey Everyone,

I've noticed that on a number of CDs containing mono recordings, there is a difference between the two channels. These are not duophonic mixes, but one channel is often very slightly louder and different to the other... Does anyone know the likely reason for this? It was my assumption that the left and right channels would be digitally identical, and I can't think why a non-duophonic CD would be mixed any other way.

Thanks!

Mono on CD - Channels Different?

Reply #1 – 2009-12-10 15:07:25

How about playing back a mono master tape through stereo heads?

Mono on CD - Channels Different?

Reply #2 – 2009-12-10 15:18:38

I didn't realise that was possible, could you please elaborate on how that works? I assumed a stereo magnetic tape would be designed such that the two channels are read independently through separate heads, whereas a mono tape would be of a different design and/or readable by only one head.

Mono on CD - Channels Different?

Reply #3 – 2009-12-10 15:38:55

Whatever the original source (record, master tape), if it was captured with a stereo pickup and/or some analog stages, then the channels won't be digitally identical.  The producer could certainly make it so on the final product, but didn't.

Mono on CD - Channels Different?

Reply #4 – 2009-12-10 15:41:53

For some (not all) tape formats there is a mono version with a wide recording head, and a stereo version that records two narrower stripes within the same area as one mono stripe. The idea was to allow some intercompatibility so that you could play a stereo tape in a mono player and the two tracks would get mixed down in the playback head, or play a mono tape in a stereo player and get two nearly identical channels. Cassette tape is an example of such a system.

Mono on CD - Channels Different?

Reply #5 – 2009-12-10 18:55:30

I don't have that many mono CDs, but as far as I know I've never run across that problem. 

I have ripped the audio from a couple of mono DVDs.  Usually it's recorded as 2-channel LPCM or 2-channel Dolby, so I have subtracted the channels just to confirm that they were digitally identical.

If the difference is noticable, I'd say it's a mastering defect that can be easily fixed.  (Sometimes, stereo has been synthesized from mono, and usually that's impossible to undo.)

Mono on CD - Channels Different?

Reply #6 – 2009-12-11 07:14:37

Interesting, thanks for the info guys. I've checked and there is definitely a difference on more than a few of my mono CDs. I find it surprising they didn't fold down or make use of both to create a better digital master!

Nevermind, thanks!