[MIC TECH ANALYSIS 6]: RCA "Living Stereo" vs TELDEC - ("large AB" vs "small AB")

Описание: What you are seeing is NOT A FREQUENCY SPECTRUM ANALYSIS, but the Frequency-dependent Cross-Correlation “FCC” for the two channels of a stereo recording, as measured by MAAT’s “2BC multiCORR” cross-correlation VST-PlugIn.

Part 1: “large AB” + few spot-mics
Dvorak’s “New World Symphony”, (RCA “Living Stereo” recording, 1957)
Part 2: “small AB” + many spot-mics
Mahler Symph. No 9 (TELDEC by Eberhard Sengpiel, 1994)

According to Eberhard Sengpiel (UdK, Berlin) only “small AB” microphone spacings of approx. 50cm provide ‘correct’ signals in respect to psychoacoustics.
On the other hand, John Pfeiffer, the record producer for the RCA label, made a judgement “by eye and ear”, when deciding for a “large AB” style microphone spacing for the – back then - new challenge of STEREO recording.

After the Dvorak-clip, the mic setup of another RCA recording from around that time is displayed: a large AB pair of Neumann M-50 omnis as main microphone and a pair of RCA 77-DX ribbon microphones (figure-8 or ’cardioid’) for the rear of the orchestra.
The graphic after that shows a schematic of a “large AB” (omni-capsule) main microphone pair, indicating the areas of orchestra musicians seated inside of the “reverberation radius”, which is typically about 5-6m for a concert hall similar in size to the Boston Symphony, Amsterdam Concertgebouw or Vienna Musikverein. Placing a mic inside the reverb radius of an instrument results in direct-sound dominating over diffuse-sound at the microphone, while placing it outside the reverb-radius will yield a rather “wet” sound with a dominance of reverb.
Now for the recording situation it makes sense to visualize the reverb radius around the microphone, as sound sources outside of it will be picked up with more diffuse- than direct-sound.

With ‘small AB’, sound sources outside the reverb radius will not be localized properly since the diffuse sound dominates the direct sound. As consequence, the need for the use of spot-microphones arises. In case of a ‘large AB’-system amplitude differences, as well as the much larger time-of-arrival differences allow proper localization even for sound sources outside of the reverberation radius, which is a clear advantage.

When comparing the “large AB” mic-schematic to the “small AB” schematic (after the TELDEC Mahler excerpt) one thing is quite evident: while for “large AB” the grey areas cover about two thirds of the string section), for “small AB” only the strings in the vicinity of the conductor will be picked up with a healthy direct-to-diffuse sound ratio … hence the need for many spot-mics for the other instruments.

And there is another advantage of “large AB” over “small AB”: as can also be seen in the video clip’s real-time analysis of correlation over frequency, large capsule spacings - as applied with the RCA Dvorak - result in highly de-correlated microphone signals (also at frequencies below 500Hz), which is vital for good spatial impression, as we know from psychoacoustic research by various scientists (see lit. references below). “Small AB” capsule spacings result in a high degree of signal correlation for low frequencies, as can be seen in the simulation right after the TELDEC clip: with a capsule spacing of 40cm signal-correlation converges towards 0.9 (becoming almost monophonic), below a frequency of 200Hz. Therefore “small AB” is certainly not suited to evoke convincing ‘spatial impression’ in the listener …

The sonic differences between the RCA and the TELDEC recording are quite audible in my opinion: while the RCA is quite ‘open’ sounding, giving a natural sense of ‘spaciousness’, with the TELDEC recording one hears mainly the sound of the spot-mics. When listening carefully to the Mahler, you will notice that there is no ‘overall’ perspecive of a large symphony orchestra, as there is hardly any 'stage depth' depicted in the mix. With the spot microphones at similar distances from their instruments, the arising sonic picture is like musicians almost "sitting on each other's lap" ...

In that respect MAAT’s “2BC multiCORR” also helps unveil the fact that the sound of the TELDEC recording - in essence - results more from a so-called “multi-microphone” mix, than a “small AB + spot-mics” mix, as can be seen by the absence of correlation larger than 0.6 which would have to show up at least below 200Hz, depending on the exact capsule spacing used:
While the “large AB” RCA recording exhibits almost complete de-correlation over the entire frequency range (as can be expected due to the capsule spacings involved), the TELDEC recording visually settles for an average cross-correlation value of around 0.3.

Much more info on microphone systems can be found in this publication on
SPRINGER NATURE: E. Pfanzagl-Cardone “THE ART AND SCIENCE OF SURROUND AND STEREO RECORDING”
https://www.springer.com/de/book/9783...

literature ref. on “spatial impression”:    • NEVATON-BPT Concert for Piano and Orchestr...