New Accession MC Feedback and Electronically Repro

Mono is best left as mono because once it is manipulated to sound stereo, the damage cannot be undone. It cannot be put back to mono by adding left and right because the additive frequencies become four times the subtractive frequencies' power.

In a stereo recording, the additive frequencies appear double the subtractive frequencies, and properly recorded will reproduce to sound natural.

Play the same stereo recording using a mono sum switch, and the additive frequencies (the central image) over-dominates, with panoramic images vastly subdued, often resembling a "tunnel" or "submarine" sound effect.

The dawning of stereo records led mono AM broadcasters to experiment with true mono. Unfortunately, the left and right sum add to give +6dB and not the +3dB the stereo effect produces through loudspeakers. The only way to add the left and right signals and result in +3dB is to push the phase of one channel out by 90 degrees.

The 90-degree phase shift is achieved by an all-pass filter but is only accurate at its tuned frequency and reverts to 0 or 180 degrees within a decade. This meant that both left and right signals had to be all-pass filtered with the tuned frequencies stepped apart a decade.

The result satisfies the power level problem, so the mono reproduction appears to share the same power levels as it does in stereo.

However, the phase displacement adds a distortion that musicians know as flanging, most evident on guitar and synthesizer fuzz effects.

There is no analogue electronics solution to this problem, but it might be solved with DSP.

Broadcast audio equipment manufacturers introduced true-mono converters to their catalogues, and it was evident to me which AM stations were using them. The Hallam mono feed was middle-heavy, and I eventually found they used A+B/2 summed mono, which confirmed it.

Other broadcasters, such as BBC mono stations, sounded more natural (within the limited bandwidth). The overseas blocking transmissions of the Wilson era illustrated the hard phasing of stations like Luxembourg and Caroline.

OK, too much information, but justified here, I think.

Graham Slee wrote: Mono is best left as mono because once it is manipulated to sound stereo, the damage cannot be undone. It cannot be put back to mono by adding left and right...

Some original recording stereo mixes from mono masters were intentionally done by the original artist as that was a big deal in the 1960's.