the 50-50 Line: The End of Mono Recording
Assembly of master tapes entailed compiling a tracklist into a final reel, which sounds simple, but in practice it also meant editing the tracks themselves. Each track master included splices and cuts to stitch together the polished performance, sometimes requiring sections of multiple takes to do so.
So you might envision the problem created by parallel mono and stereo tape paths: all of that editing would have to be done twice. To counteract this doubling of time and money, engineers developed mic-ing and recording techniques to correct for summation phase errors and permit a “50-50” mono mastering process. This meant recording only in stereo, and that stereo master tape could be folded down to one channel to cut mono LPs. Like the introduction of Contemporary’s banner jackets and similar hybrid design schemes used throughout the industry, unifying master tapes into a single set could make the editing and mastering processes more efficient, elegant, and exact.
Contemporary’s eventual use of 50-50 has been confirmed by some modern mastering engineers, including Bernie Grundman while holding the tape box for Art Pepper + Eleven on camera in 2022. Contemporary’s “50-50” two-track master tapes were assigned two numbers:
a stereo LKS xx number (A side of Art Pepper + Eleven was given LKS 89) and
a mono LKL 12-xx number (the exact same tape is also LKL 12-259).
We can see this documented on the 1961 tapes for 5006 Sounds Unheard Of:
So when did Contemporary stop recording in mono and make the switch to 50-50? We’re going to attempt to nail that down. For some context, far-better-documented engineer Rudy Van Gelder, working out of his parents’ living room in New Jersey, made the switch in November 1958. Was there any kind of bi-coastal information exchange here?
We already have the concrete evidence that Art Pepper + Eleven is a 50-50 title, recorded from March to May 1959. So we know the 50-50 line was prior to spring 1959. That’s specific enough for most people, but let’s see if we could do better.
We have a lot of data to pull from. From our discussion of Tape Numbers, we know that mono and stereo tapes were assigned numbers sequentially in their own columns, such that stereo LP tape numbers lagged well behind their mono counterparts, which had a two year head start. For titles recorded in the dual-tape-deck era (recordings made to both stereo and mono), there is a lot of randomness in the stereo tape numbers due to those stereo assemblies being made at various points independent of when the material was actually recorded.
However, after the 50-50 line, with stereo and mono numbers assigned to the exact same tape meant that those numbers would also be assigned at the same time. So, as a tape was assembled, the stereo tape count would go up by 1, as would the mono. So — theoretically — after the 50-50 line we should see a very consistent gap between stereo and mono tape counts, one that doesn’t jump and fall and change drastically title to title.
Theoretically. Let’s try to visualize this and see what it tells us. That gap between mono and stereo tape numbers for the same title is what we’re going to use. We can compare each title’s stereo tape numbers AGAINST its mono tape numbers and then comparing those comparisons.
For our dataset we’re including every title released in stereo and mono, so catalog numbers 3519 to 3616. But we also need remove outliers or potential data mines. Right away the Belated Twelve are gone from the dataset, as is late mono-only release 3584 and New York Hentoff recordings 3552, 3554, and 3562.
Then let’s subtract the LKS xx stereo number for each Side A from the LKL 12-xx number for the same side.
As an example, take Way Out West:
The stereo side A tape is LKS 33.
The mono side A tape is LKL 12-127.
Subtract stereo from the mono. 127 — 33 = 94.
So 3530 Way Out West has a mono/stereo tape number gap — or tape delta, which sounds cooler — of 94.
The tape delta for each CR title 3519 to 3616 (minus outliers) is plotted in the graph below.
This makes it pretty obvious that there was a change made, after which the tape deltas go essentially flat, but for the outlier dip or mini-peak.
But consider a fundamental error in our presentation here. Any change in recording technique — eg. the 50-50 line we’re trying to find — would correspond with a specific recording date. Right now our X axis is still showing catalog numbers. Catalog numbers will have a direct relationship to date but don’t sequence perfectly with them.
So let’s reorder our data. Using the latest recording date for each title, our graph is arranged as below:
Doesn’t change much! But we can rest easy knowing we’re using the data right.
Using time as our X-axis squishes the 1950s recordings to the far left because the frequency of new recordings decreased considerably in the 1960s. That explained, we see a pretty obvious transition between chaotic tape deltas in 1956, 57, and 58 before a distinct leveling off afterwards.
We do have some outliers putting dips and peaks in our flatline; these are two LPs compiled/released three years after their recording (3607 and 3613) and one which has a wacky low mono tape number (3606) for I know not why. These don’t impact the 50-50 line question, but removing them will allow us to visualize things a little better. Specifically, it will allow us to see how belated stereo tape assemblies of 1956-58 titles impacted tape delta.
Not to get too deep into this, but you can see two slopes on the graph that point to where assembly of old stereo tapes (The Belated Twelve) caused LKS xx tape numbers to gain ground on LKL 12-xx tape numbers… and thus, caused the tape delta to decrease:
We could dissect that in more detail. Let’s not. Keeping focus on the main event:
Let’s shorten our timescale so we can see some granularity in the late 1950s and draw ourselves a 50-50 line.
Looking closely and making a tiny judgment call, our line lands between Swingin’ the 20s (recorded Nov. 2, 1958) and King Size (November 26, 1958).
And just like that, we arrive at a theory: Mono recording ended in mid-November 1958.
All of the research I’ve laid out and dissected in good faith. However, in that vein, I should acknowledge the confirmation bias at play that gets us to this November 1958 dating.
We are choosing to assume this deduced “50-50” line is the real thing. Understand that the market was changing rapidly in this period, so another scenario is possible: that the growing stereo LP market encouraged Contemporaary to release all new titles in stereo, which prioritized stereo tape assembly to the point of near-concurrence with mono tape assembly. That business decision may have come months prior to the technical decision to end mono recording, but the former could create the illusion that the latter happened in November 1958.
I don’t think that’s what happened, but the data leaves room for that ambiguity. For example, 3574 Carl’s Blues (1960 tape assemblies of 1957/1958 mono/stereo dates) and 3589 For Real! (1961 assembly of a March 1958 date) show this, where the maintenance of a stable tape delta might have been due to a) the creation of 50-50 tapes, or b) the consciously simultaneous creation of separate mono and stereo master tapes.
If separate stereo and mono tapes were assembled at the same time (or in immediate succession), tape deltas would stay constant and act a lot like a unified master tape sporting both LKS and LKL numbers.
For our purposes, we’re going to run with the mid-November 1958 separation until shown reliable evidence to the contrary.