Vinyl cutter head equalisation ..

During all those trials and tests we conducted while optimizing our vinyl cutter head equalisation electronics, we found a real parallel between: 1) A cutter head and it’s vinyl record. &  2) A loudspeaker and it’s listening room. Both the implications and the way this parallel was discovered is all to do with EQ. For loudspeakers, particularly studio monitoring speakers, it has been (rightly so we think) frowned upon to EQ the system to account for, and correct issues that, are more likely born of room acoustics. Indeed, since room acoustic issues are the product of real physical things, any amount of electronically shaping the sound can’t alter things in the “physical domain”, and so, can’t be right !.

However, EQ’ing the speaker itself is a different thing.

In that, we are trying to get the “best” “flattest” or possibly “most musical” sound quality from a speaker system without any thoughts of what a room (or different rooms) will then do to it. Bose are probably the speaker manufacturer most prolific and experienced at EQ, and prove the point very well. Their many speaker systems based on multiples of small “full range” drivers (802, 502 and so on) employ quite harsh EQ to get the things to sound right. Quite nicely turing accepted speaker design on it’s head, they don’t start from the point of having a “flat” speaker driver, but one which definitely isn’t flat, and which exhibits the other design qualities they are looking for. The way in which this type of speaker corrective EQ is done: analog, digital, active, passive Etc. becomes part of the speaker design, and part of the speaker systems inherent sound. So. lets say, a speaker uses an active analog EQ to correct and extend the speakers high frequency response to a desired level. That EQ electronics may well have “side effects” like, say, a bit of phase distortion, which could adversely affect sound quality, or, become “designed in” as part of a speaker’s “character”.  This is, in effect, no different to using an overdriven valve amp to induce “that certain sound character”, so, can in no way be seen as “wrong” or technically incorrect, although most would agree, not exactly “pure as the driven snow” !.  Of course, a digital equalizer will implant less “sound character”  But, the point is, that EQ on “a piece of equipment” is absolutely fine, refines that piece of equipment to the standard, or sound quality the designer was after, and can remove little niggles of unwanted sound character that are often dictated by the “electromechanical” limits of that piece of equipment. Very true in loudspeaker drivers, and equally in record cutting heads !. Attempting to EQ a speaker system in, and to, a room however, is another kettle of fish !. As we’ve already mentioned, EQ can’t change the physical attributes of a room and this fact leads to it being possible to set EQ’s several different ways while still getting the same acoustically measured frequency response.

vinyl cutter head equalisation example graph.jpg

Here’s an example of 2 vinyl cutter head equalisation settings that both gave a flat response with real time measured pink noise. The RED trace is one setting, the BLUE trace, the second setting.

Vinyl cutter head equalisation …

Is an alarmingly similar beast to a loudspeaker driver, both have driver coils, and where the speaker has a cone, the cutter head has a stylus suspension system. Both cones and suspension systems are physical, so they both have mechanical resonance’s and limits. The moral, ethical, and technical acceptability of EQ on a cutter head is then the same as for a loudspeaker: Adjust and “trim” to correct for the cutter head’s own frequency deviations is fine. But, the huge complexities of the fact that the cutter head is just as dynamic a device as a loudspeaker: ever changing it’s electrical and electro/mechanical properties with temperature, closeness to physical limits and so on, along with the physical properties of the actual vinyl being cut, and possibly even the dynamic effects of the cutter lathe’s own mechanical components, makes the “signal in” to “playback from cut vinyl”, as an entire “audio journey” rather like a speaker in a room. Just like a speaker in a room, this gives rise to the situation where, more than 1 EQ setting will get you the same frequency response. Also, just like a speaker in a room, different EQ settings may get you the same measured frequency response, but, every setting, will get you a completely different “sound”. We have several cutter head EQ settings, and, while all “flatten” the response to within very similar limits, the settings are wildly different. When you consider the complexity of all the elecro, and electromechanical dynamics going on in the cutting process (just like a speaker in a room) it’s too be expected !!. The trick is to use the resulting sound differences to every ones advantage, enhancing the already appreciated “vinyl” sound that little bit further if at all possible. We have 2 settings in particular that demonstrate the “chalk n cheese” sound qualities produced by the 2 “chalk n cheese” EQ settings, but which both measure the same ! On a good monitoring system, with decent phase integrity, an adequate frequency response, and reasonable “imaging” we can “sink” a vocal back into the track, or pull it 4 feet forward into the room – just with the choice of EQ setting, nothing more !. This staggering sound quality difference, where not just stereo image, but, even, 3 dimensional image is affected can’t possibly be down to a “bit of tone control” only. More like, the knock on effects that “tone control” has on just about every aspect of recorded sound when your dealing with a highly dynamic electro mechanical recording system. Not something you could ever “design” either, but the subject of informed listening and tweaking !. Our conclusions are:
* It’s ok to EQ bits of equipment individually, or as a system chain to enhance their performance.
* It’s NOT ok to EQ a “room” – that problem should be dealt with “at source” (ie: physically)
* It’s very very ok to listen, tweak and play. It can be fun, musical, creative and uncover things that will make a musical performance nicer, more involving, even sexier !