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VCA vs. Opto compression
Old 13th March 2008
  #1
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pootkao's Avatar
 
🎧 10 years
VCA vs. Opto compression

Hey folks
Just wondering what the technical differences are between the two design styles? How does each work? Which usually gets used for what? I had someone ask me today and I didn't know the answer.

I searched through a couple pages using the search function, but nothing came up.

ps: I'm NOT interested in which you prefer, just in their differences in processing -- which is why I'm putting this in the tech forum anyways.

Thanks for your help!
Old 13th March 2008
  #2
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Chaellus's Avatar
 
🎧 10 years
here you go taken from wikipedia and UAD



Optical compressors -use a light sensitive resistor (LDR) and a small lamp (LED or Electroluminescent panel) to create changes in signal gain. This technique is believed by some to add smoother characteristics to the signal, because the response times of the light and the resistor soften the attack and release



The VCA's (Voltage-Controlled Amplifier) gain is determined by the voltage supplied to a control input. Virtually every compressor technically uses a VCA, whether it is based on FETs, variable-mu tubes, or optical elements. However, in the parlance of audio, the term “VCA compressor” usually refers to a compressor using an integrated-circuit VCA for gain control. Integrated VCAs have the desirable properties of low distortion, high input impedance, low output impedance and high bandwidth. In addition, integrated VCAs provide precise relationships between control voltage and amplifier gain. Because of these features, integrated VCAs can be used to modularize the design of compressors or other gain control devices: High input impedance allows signal detection to be performed without worrying about loading the detector circuit. Low distortion and high bandwidth have obvious advantages. Most importantly, the tightly prescribed relationship between control voltage and amplifier gain allows for conscious design of compression curve shapes. By contrast, with most vintage discrete compressor designs, the form of the compression curves depends mostly on the vagaries of the devices employed to provide gain reduction. VCAs usually have an exponential dependence between control voltage and gain, so that the sensitivity of the amplifier with respect to the control signal is specified in dB/V. This type of VCA is said to have “logarithmic gain control.” The logarithmic-gain-control VCA is ideal for compression, because any constant compression ratio can be achieved by using the scaled, log-encoded signal level as a control signal for the VCA.
Old 13th March 2008 | Show parent
  #3
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Chaellus's Avatar
 
🎧 10 years
the LA-2A is an Optical Compressor and the DBX 160 is a VCA compressor

optical compressors are more light on subject material usually ....no pun intended, then VCA's are usually more aggresive as well...so according to that may be good for something better than others dont for get about terms such as hard knee and softknee compression thats also useful to know
Old 14th March 2008 | Show parent
  #4
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pootkao's Avatar
 
🎧 10 years
Awesome, thanks!

Could you also give me a little description of the differences between FET, opto and Vari-Mu. I know the sonic differences (usually ) but am curious as to how each one works differently.

Thanks for humoring a technical idiot.
Old 14th March 2008 | Show parent
  #5
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🎧 10 years
also taken from the manley site and nrg recording site.....



variable-mu - "Mu" is tube-speak for gain, It works by using the "remote cut-off"
or re-biasing of a vacuum tube to achieve compression.




FET-(Field Effect TRansistor)- Compression is achieved by a Field Effect Transistor (FET) which is used as a variable resistor. the FET acts like a resistor whose resistance is controlled by the voltage applied to its gate. The higher the voltage applied to the gate, the smaller the drain-source resistance will be.the greater the voltage applied to the gate of the FET, the less resistance, hence large signals cause the FET to reduce the gain.
Old 14th March 2008 | Show parent
  #6
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🎧 10 years
Can you go into greater detail on the Vari-Mu? (I knew Mu was tube, by the way, at least I'm not a total writeoff...)

How does the ratio affect the tube re-bias? How does that system work?
Old 14th March 2008 | Show parent
  #7
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🎧 15 years
Hi
The 'gain' of a vari mu valve changes depending where you are working along it's 'load line'.
For example a 1 volt change in grid voltage may produce 10 volts of variation near one end of the range, but a 1 volt change may produce a 20 volt variation further up the operating range.
All valves have a non linear response but those selected for 'vari-mu' service are more extreme. They were originally designed for RF applications where distortion was not important in the same way as audio (Automatic Level Control).
This is non linear and produces significant distortion but by using it in 'push-pull' and applying various other 'tricks' can produce a stage capable of reasonable but audio pleasing performance.
A FET can also exhibit similar non linearities and the methods of counteracting these differ.
Matt S
Old 24th October 2012
  #8
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fuzulu's Avatar
whats RF applications ?
Old 24th October 2012
  #9
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fuzulu's Avatar
what king of compressor is JDK R22 , VCA or something else ?

I know that Alesis and Api2500 are both VCA , so what makes the price diff in these compressors ?

whats kind of compressor is dbx 166XL

thanks !!
Old 25th October 2012 | Show parent
  #10
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🎧 10 years
Quote:
Originally Posted by fuzulu ➡️
whats RF applications ?
Radio Frequency
These tubes were used in TVs
Old 25th October 2012
  #11
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🎧 15 years
Hi
As Jensenmann says, TVs but also radios and probably other gear as part of the automatic level control circuitry (ALC) which ensured that your radio gave a similar audio level when tuning to a local or remote radio station. The ALC would typically have been in the RF section not AF.
The demand for 'high quality' AF level control was not huge in the grand scheme of things so there are relatively few valves specifically designed for this purpose.
DBX and API are VCA designs. DBX was one of the 'originators' of the integrated circuit version of VCAs but now name change to THAT Corp.
A VCA of this 'integrated transistor' design CAN and was made using a pile of seperate transistors (Alison Research, Valley People, the black, then gold canned DBX modules etc) but while 'good' at a specific temperature and voltage environment would 'drift' giving quite wild gain and distortion changes. Putting them into an IC allowed MUCH better performance and predictability.
Try reading the notes on the THAT corp website and you will get some idea of what goes on.
Difference in price of gear is partly 'fashion'. The actual 'elements' doing the compression are relatively cheap but the surrounding circuitry will influence the price considerably, especially if high quality transformers are involved. A modern THAT corp VCA chip is a few Dollars apiece but by the time it is housed in a fancy box with the support electronics you can have huge range in cost.
Going a bit off topic, the A/D chip that can process 24 bit 192K costs about 5 Dollars in quantity, but MUCH is spend providing it a good 'home' which is why the better 'boxes' cost so much.
Matt S
Old 25th October 2012
  #12
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JohnRoberts's Avatar
 
🎧 10 years
Good information, be careful to not read too much into the method of gain manipulation. All dynamics processing involves a gain element (Like the VCA) and a side chain, or control voltage path.

The sound result is a combination of how well the VCA does it's job, and what the control path tells it to do.

Modern VCAs are much cleaner and more accurate than early gain elements. In a few cases as already noted the natural time constants of a gain element technology complemented the sound quality. In far more cases, old technology is just old, while in recording people seem to cling to old sound signatures, longer than they deserve.

Do not assume that all dynamics units using the same technology will sound and act the same.

JR
Old 25th October 2012
  #13
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fuzulu's Avatar
thanks for the info guys , its interesting that the electronics & circuitry holding the vca chips are what actually increase the cost a lot . so basically the circuitry in an api is really good , I would like to know more on this if possible , like what actually in the circuit increases cost , is it just the quality or is it because of extra features , and some info on transformers , why do they increase the cost , what do they add to the sound ??
Old 25th October 2012 | Show parent
  #14
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🎧 10 years
Quote:
Originally Posted by fuzulu ➡️
thanks for the info guys , its interesting that the electronics & circuitry holding the vca chips are what actually increase the cost a lot . so basically the circuitry in an api is really good , I would like to know more on this if possible , like what actually in the circuit increases cost , is it just the quality or is it because of extra features , and some info on transformers , why do they increase the cost , what do they add to the sound ??
Price/cost of commercial products is directly increased by component cost, and inversely affected by production volume. So a product may be expensive because it uses very expensive parts, or because it only sells a few.

JR
Old 25th October 2012
  #15
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🎧 15 years
Hi
The TOTAL sidechain is possibly the most determining part of the 'sound' of a compressor. This is the time, ratio linearity and other factors that actually 'process' the audio signal. In a opto type unit, the LA2A for example the audio signal is fed into a luminescent display which is quite fast reacting. The light dependant resistor is pretty slow and has an even longer 'release' characteristic. A transistor or FET element could act at the same 'speed' as the audio signal (although it would sound horrible if it did) but it's OVERALL characteristics can be readily manipulated by a bit more electronics (attack release etc.) Arguably this would make it a more useful unit as it is not 'fixed' in hardware. The LA2A of course has a valve / transformer signal patyh so it will cound different to most even when not compressing.
You also have issues such as RMS or peak rectification which affect the sound.
There are so many aspects to the 'cost' of a unit which like many things bear little relationship to the real cost of the 'active' element.
A pair of 'Gucci' sunglasses cost the practically same to make as your cheap 'supermarket special offers'. There was an interesting TV programme in the UK which showed the SAME production line making many of the 'expensive brand' sunglasses.
Matt S
Old 25th October 2012
  #16
Gear Head
 
🎧 5 years
Optos and tubes react much more slowly than fet and vca comps. this can be good,or bad depending on the situation. Tubes are known to add "vintage" warmth to sound, where vca/fet are known for being quick clean and accurate.

Sent from my SPH-M820-BST
Old 25th October 2012
  #17
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🎧 15 years
Hi
No, Valves will (or can) act at the same 'speed' as FETs and transistors ((ICs).
In the past they probably were used with a sidechain that is relatively 'slow' because super fast was not deemed necessary or they tried to avoid an obvious 'problem' that very fast 'attack' rates can appear as a 'tick' or 'thump' because valves are pretty 'wild' in terms of constant characteristics as they 'drift' with supply voltages, temperature and time.
A valve signal chain can have distortion WAY beyond what is deemed acceptable nowadays and treated as a single element in a music 'mix' is perhaps prized as part of 'the sound'. The LA2A and other units designed as compressors for radio were OK for general use because transmitters had HARD limiters for protection. Overmodulation of a powerful transmitter is not permissable as it can quickly lead to catastrophic failure.
Matt S
Old 25th October 2012 | Show parent
  #18
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nosebleedaudio's Avatar
 
🎧 15 years
Quote:
Originally Posted by fuzulu ➡️
thanks for the info guys , its interesting that the electronics & circuitry holding the vca chips are what actually increase the cost a lot . so basically the circuitry in an api is really good , I would like to know more on this if possible , like what actually in the circuit increases cost , is it just the quality or is it because of extra features , and some info on transformers , why do they increase the cost , what do they add to the sound ??
Several "parts' can add to the cost, some pots are less than $1.00 ea. the Clarostats 381 series I use cost around $22.00 ea...
Switches are the same..8 to 10 times the cost..
Transformer or Transformerless?? Huge difference in $$.
Connectors, how they are mounted, grounding ect..
That Corp makes several levels of VCA's, the top level cost $$ and outperforms the less $ ones...I use the 2181A.
Old 25th October 2012 | Show parent
  #19
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nosebleedaudio's Avatar
 
🎧 15 years
Quote:
Originally Posted by LoveAudio ➡️
Optos and tubes react much more slowly than fet and vca comps. this can be good,or bad depending on the situation. Tubes are known to add "vintage" warmth to sound, where vca/fet are known for being quick clean and accurate.

Sent from my SPH-M820-BST
Sorry but not correct.
Old 25th October 2012
  #20
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🎧 15 years
Hi
If you really analyse the performance of even the excellent 2181A VCA chips there are 'artifacts' that are present due to the internal use of NPN and PNP transistors. In a 'compressor / gate' application this should be corrected which requires extra components and design. This is pretty subtle and usually much less than using other processes excepting opto.
Matt S
Old 25th October 2012
  #21
Gear Head
 
🎧 5 years
Quote:
Originally Posted by nosebleedaudio ➡️
Sorry but not correct.
How so? Tubes add even harmonic distortion, hence tube warmth. tubes have a slower response by nature than fet comps. Optos also have slow response compared to fet and vca comps.

Sent from my SPH-M820-BST
Old 25th October 2012 | Show parent
  #22
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nosebleedaudio's Avatar
 
🎧 15 years
Quote:
Originally Posted by LoveAudio ➡️
How so? Tubes add even harmonic distortion, hence tube warmth. tubes have a slower response by nature than fet comps. Optos also have slow response compared to fet and vca comps.

Sent from my SPH-M820-BST
Opto are used for VERY HF info...
Tubes are used in UHF Transmitters..
Its the circuit that "Controls" the VCA is what makes it respond slower or faster....
Old 25th October 2012 | Show parent
  #23
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JohnRoberts's Avatar
 
🎧 10 years
Quote:
Originally Posted by Matt Syson ➡️
Hi
If you really analyse the performance of even the excellent 2181A VCA chips there are 'artifacts' that are present due to the internal use of NPN and PNP transistors. In a 'compressor / gate' application this should be corrected which requires extra components and design. This is pretty subtle and usually much less than using other processes excepting opto.
Matt S
THAT corp engineer Gary Hebert wrote an AES paper a few years back describing the inner workings of their latest generation VCA. There is an issue with current ratioing VCAs from beta mismatches between NPN and PNP pairs. This generates a subtle nonlinear current error at the bases between top half of audio waveforms and bottom half.

The improved process THAT is using deliver better matched devices, among other improvements.

For any dynamics processing, artifacts from the side chain gain manipulations that effectively perform a multiply to the audio waveform will surely swamp the VCA non-linearites.

I guess a digital multiply performed inside the digital domain is about the only thing cleaner, that offers useful (accurate) control, but that too will suffer from multiplication products.

I expect opinions may vary.

JR

.
Old 25th October 2012
  #24
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🎧 15 years
Hi John
I was referring to differing base to collector or emitter capacitances which can 'feed through' 'ticks' when you have a very fast control voltage change. I spoke with Fred Floriou (apologies if his name is spelt incorrectly, it has been a while!) and developed a 'work around for this situation, successfully used at AMEK.
There are undoubtedly other effects which may be more or less important.
Matt S
Old 25th October 2012
  #25
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fuzulu's Avatar
u guys are pretty hardcore , thanks for the info ,

does anyone know what kind of compressor is Jdk R22 , is it a vca ? I know it uses some chips ...
Old 25th October 2012
  #26
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ksandvik's Avatar
 
🎧 10 years
If you have Logic or Ableton Live it's a no-brainer to try each one out and hear how they sound.
Old 26th October 2012 | Show parent
  #27
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JohnRoberts's Avatar
 
🎧 10 years
Quote:
Originally Posted by Matt Syson ➡️
Hi John
I was referring to differing base to collector or emitter capacitances which can 'feed through' 'ticks' when you have a very fast control voltage change. I spoke with Fred Floriou (apologies if his name is spelt incorrectly, it has been a while!) and developed a 'work around for this situation, successfully used at AMEK.
There are undoubtedly other effects which may be more or less important.
Matt S
If you drive the + and - control ports equal and opposite amounts, there should be a first order cancellation of charge from capacitance, while again, the matching of the NPNs and PNPs matters.

JR
Old 26th October 2012 | Show parent
  #28
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dcollins's Avatar
 
🎧 15 years
Quote:
Originally Posted by LoveAudio ➡️
How so? Tubes add even harmonic distortion, hence tube warmth. tubes have a slower response by nature than fet comps. Optos also have slow response compared to fet and vca comps.
Tubes do not add just 2nd harmonic, that's a common misconception.

Depends on the circuit.

Same with transistors.

DC
Old 26th October 2012 | Show parent
  #29
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dcollins's Avatar
 
🎧 15 years
Quote:
Originally Posted by JohnRoberts ➡️
If you drive the + and - control ports equal and opposite amounts, there should be a first order cancellation of charge from capacitance, while again, the matching of the NPNs and PNPs matters.
We used to do that back in the olden days of sample and hold stages for PCM converters before oversampling.

Charge injection was best in DMOS, if I can remember back that far......................


DC
Old 11th November 2012
  #30
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🎧 5 years
Hey, I'm wondering what you guys think about the Avalon Vt-747sp as an all around compressor for a variety of uses such as Vocals, Percussion, Bass, Synths, etc.

Thanks! ~Vatra~
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