Signal Processors II - VGAs PDF

Textbook Supplement: p. 279-284 SIGNAL PROCESSORS II - VARIABLE GAIN AMPLIFIERS SIGNAL PROCESSOR TYPES FREQUENCY AMPLITUDE TIME & SPACE DOMAIN DOMAIN DOMAIN C...

Textbook Supplement: p. 279-284 SIGNAL PROCESSORS II - VARIABLE GAIN AMPLIFIERS SIGNAL PROCESSOR TYPES FREQUENCY AMPLITUDE TIME & SPACE DOMAIN DOMAIN DOMAIN Compressors Reverb Limiters Equalizers & Expanders Delay Noise Gates Hybrid De-Essers; Noise Reduction; Distortions; Modulation (Chorus, Phase, Pitch, Tuning) SIGNAL PATH REVIEWED In-Line Side-Chain Insert/Send Aux Send/Return Receive Reverb & Delay Equalizers Compressors Dry Signal Wet Signal Limiters (Direct Sound) (Reflected Sound) Expanders Noise Gates Main Mix VGA IMPLEMENTATION IN THE RECORDING CHAIN VGA Processing Pre-Tape VGA Processing Post-Tape OUTBOARD CONSOLE PLUG-IN OUTBOARD CONSOLE Is committed to recording, not undo-able. Not committed to recording, monitoring only. Plug-In: Outboard VGA: Within DAW VGA post pre-amp, pre-tape Outboard: Console VGAs: Patched to post-tape monitor channel Implemented at pre-tape channel Console: Implemented at post-tape channel VGAS IN THE RECORDING CHAIN - FAQ “SHOULD I COMPRESS TO TAPE?” ▸ Generally, start with listening to the dynamic range of a sound without compression rst, then assess if some is needed to assist with gain structure management, or perhaps a creative implementation. “IS THERE ANOTHER OPTION?” ▸ Yes. Riding level to tape, though more active and involved, can be a very transparent way to adjust gain structure, but requires careful attention from the engineer. “HOW MUCH IS TOO MUCH?” ▸ Some engineers will avoid committing heavier compression to tape, the idea being that dynamics importantly affect the impact of a performance. Others may argue that aggressive compression is exciting and adds to intensity. Different compressors will also sound vastly different, and in uence this decision. Again, trust your ears - when in doubt, implement post-tape. “CAN I UNDO COMPRESSION?” ▸ Not really. Sometimes, upward-expanders and/or transient designers (to be discussed) can bring back some amount of dynamic range, but won’t truly “un-compress” a sound. When in doubt, try parallel. “WHAT ABOUT GATING TO TAPE?” ▸ Proceed with caution. This is not undoable - bringing lower level sounds back isn’t doable. Gentle expansion may be a more effective approach to tape. fl fi VARIABLE GAIN AMPLIFIERS (VGA) VARIABLE GAIN AMPLIFIERS (VGA) DEFINITION ▸ Devices which automatically alter their output level based on user selectable parameters. ▸ Provides automatic level changes to the output of an audio signal based on its input. Audio In Amp Audio Out Control Circuit Parameters “Detector” VGA CIRCUIT TYPES VGA CIRCUIT TYPES ▸ Different VGAs will use different electrical circuits to both detect and affect gain changes on a signal. ▸ The primary circuit types are: ▸ Voltage Controlled Ampli er (VCA) FET ▸ Field Effect Transistor (FET) ▸ Optical Cell VCA ▸ Variable Tube Plate Voltage (Vari-Mü) ▸ Algoritmic/Digital T4 Optical Cell Tube Algorithmic fi VGA CIRCUIT TYPES ▸ Field Effect Transistor (FET) ▸ Uses voltage variation in a small transistor to affect gain changes. ▸ Characteristics: Extremely fast-acting, can be aggressive sounding to neutral, dependent on settings. Neve 33609 Universal Audio 1176 VGA CIRCUIT TYPES VOLTAGE CONTROLLED AMPLIFIER (VCA) ▸ Uses voltage change in a solid state chip-based Dbx 160VU circuit to affect gain changes. ▸ Characteristics: Fast-acting, can range from transparent-sounding to aggressive. API2500 Stereo Bus Compressor SSL G-Series Stereo Bus Compressor VGA CIRCUIT TYPES OPTICAL ▸ Uses a light sensitive photocell to affect gain changes. ▸ More input = brighter light = more gain change. ▸ Characteristics: Quite transparent, not as fast response, fairly neutral sounding. Universal Audio LA3A Teletronix LA2A Urei LA4 VGA CIRCUIT TYPES VARI-MÜ ▸ Uses voltage variation in a tube’s plate-voltage (Mü) to affect gain changes. ▸ More input = higher plate voltage = more gain change. ▸ Characteristics: Slower, warmer, easy to adjust. 3 Tubes per channel. 10 Tubes per channel. Thermionic Culture Phoenix Fairchild 670 VGA CIRCUIT TYPES DIGITAL/PLUG-IN ▸ Use software algorithms to alter gain. ▸ Some intend to emulate analogue devices, others may process in ways no analogue processor could (look-ahead, instant attack times, etc.) VGA CIRCUIT TYPES DIGITAL/PLUG-INS ▸ There are many! VGA TYPES VGA TYPES COMPRESSOR ▸ A VGA whose output level increases at a slower rate than its input level. ▸ It makes loud sounds quieter, therefore making quieter sounds louder. ▸ May control the dynamic range of a signal, change the amplitude envelope (ADSR), or Output increases slower than create special effects. input. VGA TYPES LIMITER ▸ A VGA whose output level will not increase past a user selectable level. ▸ Effectively, a compressor with a very high ratio. ▸ Commonly used as a safety device, or for preventing “overs”/clipping. Peak Limiter - True Peak Output will not increase past ceiling. Output will not increase past threshold. Compresor/Limiter - High Ratio VGA TYPES EXPANDER ▸ A VGA whose output level will decrease at a faster rate than its input level. ▸ It makes quiet sounds quieter, therefore louder sounds louder. ▸ Commonly used to reduce unwanted noise, leakage, or shape ADSR. Output decreases faster than input level. VGA TYPES NOISE GATE ▸ A VGA whose output mutes once the level drops below a selectable level. ▸ Essentially, an Expander with a very high ratio becomes a gate. Output mutes below threshold. VGA PARAMETERS VGA PARAMETERS THRESHOLD ▸ The user selectable level where gain change will begin. ▸ Commonly express in negative dB values. Threshold VGA PARAMETERS RATIO ▸ The proportionate change between input and output once the threshold is exceeded. Expressed as 2:1, 4:1, etc. for compressors. ▸ Example: 2:1 - For every 2dB above threshold only 1 dB increase at output. ▸ Compression Ratios: 2:1 to 8:1. Limiter Ratios: 10:1 or higher. ▸ Ratio is reversed (1:2, 1:3, 1:10) for Expanders/Gates. Fixed Ratios Variable Ratio VGA PARAMETERS ATTACK TIME ▸ Amount of time for the gain change to occur once the threshold is exceeded. ▸ Commonly express in micro-seconds (µs) to milliseconds (ms). Attack Time VGA PARAMETERS RELEASE TIME ▸ Amount of time for the gain change to return to uncompressed. ▸ Expressed in milliseconds to seconds. Release Time VGA PARAMETERS KEY INPUT OR SIDE-CHAIN INPUT Side-Chain ▸ Uses an external input signal, not the main signal, to control the amount of compression. ▸ Often used to create program dependant automation (ducking), or frequency- dependent compression (De-Essing). Key Input VGA PARAMETERS KNEE ▸ Controls the bend of the response curve between a “hard knee” or “soft knee”. Usually measured in dB. ▸ Soft Knee: Slowly increases up to the ratio as the input increases. Helps to make compression less audible. ▸ Hard Knee: Ratio is consistent at the set amount, makes compression more audible/noticeable on ADSR of a signal. Hard Knee Soft Knee VGA PARAMETERS STEREO LINK ▸ Applies the same gain reduction to both/all channels. ▸ Helps prevent image shifting that can occur if channels are compressed individually. Link/Unlink Link Master The Loudness War - by Matt May eld Peak (pun!) loudness wars content. fi PRACTICAL VGA APPLICATION PRACTICAL COMPRESSION APPLICATION ▸ How and when to use compression can be dif cult to determine at rst, but will often fall into one of four general reasons for use: 1. Dynamic Range Control 2. Sound Envelope Manipulation 3. Parallel 4. Peak Limiting ▸ These approaches may overlap and combine depending on desired results. fi fi 1. DYNAMIC RANGE CONTROL ▸ Reduces the need for an engineer to adjust levels as regularly, when a more consistent level is needed. ▸ Due to the desired result being one that is relatively inaudible, more subtle settings. ▸ Commonly employed to tape when signals have a wide dynamic range. ▸ There are no speci c settings that be universally applied, as all sounds are different, but some general guidelines: ATTACK TIMES ▸ Set to roughly follow the natural attack or transient nature of the source signal. ▸ Faster transient? Faster attack time. Slower transient? Slower attack time RELEASE TIMES ▸ Gain Reduction should return to zero prior to the transient of the next note. ▸ Faster tempo? Faster release time. Slower tempo? Slower release time COMMON RATIOS ▸ Compressor type and source dependent, often around 1.5:1 - 4:1 COMMON GAIN REDUCTION (GR) ▸ Compressor type dependent, -1 to -6 dB Gain Reduction at peaks. COMMON USES ▸ Piano, acoustic guitar, clean electric guitar, background vocals, bus/group compression. fi 2. SOUND ENVELOPE MANIPULATION ▸ Manipulates transient content and characteristics of a signal, i.e. ADSR ▸ Helps to contain or shape an element in a mix by de ning a space/size for said element. Hearing the artifacts of compression is more desirable in this application. ATTACK TIMES ▸ Used to round off (if faster) or enhance (if slower) the leading edge or transient of a sound. ▸ Faster attack time = Subdues the attack of source. Slower attack time = More attack/punch allowed through. RELEASE TIMES ▸ Can be used to shape the decay to t the desired length/control. ▸ Faster release = more decay, sustain, and release brought up. Slower release = less length and sustain brought up; less noticeable. COMMON RATIOS ▸ Compressor/source dependent, usually 4:1- 20:1 COMMON GR ▸ Compressor dependent, often -5 to -10 dB Gain Reduction COMMON USES ▸ Electric Bass, aggressive/loud vocals, individual drums, acoustic or clean guitars, synths, anything needing transient shaping. fi fi 3. PARALLEL COMPRESSION ▸ Mixes a combination of a heavily compressed signal with an uncompressed sound. Routing is not inline - a bus or aux send. ▸ Can be fairly inaudible, despite heavy processing, as the original transient content is still in tact. ▸ Brings out lower level detail - this may increase noise, bleed, etc. ATTACK TIMES ▸ Often set very fast in order to minimize transients. ▸ Faster attack time = more “squash” to the signal. RELEASE TIMES ▸ Again, often very quick in order to bring up everything after the transient to roughly the same level. Faster release = more low-level content brought up. COMMON RATIOS ▸ The higher, the more the compressor’s character is brought out; anywhere from 2:1 - ∞:1. COMMON GR ▸ Whatever works for material; often quite a lot - a compressors character will be apparent with heavy GR. COMMON USES ▸ Kick drum, snare drum, bass, lead vocals, anything requiring thickening or enhancing of character. 4. PEAK LIMITING ▸ A.k.a. Brick Wall Limiting. Now in digital metering often referred to as True Peak Limiting. ▸ Most often a digital compressor with a very aggressive ratio, attack, and release time. ▸ Designed to remove peak information as quickly as possible while simultaneously boosting the average level. The result is as loud as possible without exceeding the headroom of the recording medium. ATTACK TIMES ▸ Often microseconds, or, if set to look-ahead, faster than the transient in order to prevent “overs”. ▸ On many peak limiters, the time of the attack is not continuously adjustable, but preset. RELEASE TIMES ▸ Almost always very fast. With this type of limiting, slower release times make the limiter less effective. Commonly ranges from 0.1 ms to 1 second COMMON RATIOS ▸ Very high ratios, designed not to let the transient pass. Often ∞-1 COMMON GR ▸ Digital clipping usually becomes audible after 2-4 dB of Gain Reduction, limiter dependent. COMMON USES ▸ Most often used only after mix-down, best left to a Mastering Engineer - not recommended until this stage. UNLOCKING YOUR COMPRESSOR EDITED EXCERPT FROM “MIXING WITH YOUR MIND” BY MICHAEL PAUL STAVROU UNLOCKING YOUR COMPRESSOR ▸ As a starting point, it’s useful to hear what a compressor is doing with each parameter. ▸ Before beginning: Set Attack and Release times as quick as possible, Ratio high (6:1 or higher), and threshold so that gain reduction is constant. ▸ This allows focus on one parameter at a time. STEP 1 – SETTING YOUR ATTACK TIME ▸ Release Time at minimum; Ratio at maximum; Threshold at low (gain reduction constant) ▸ Tune your ears to: Listen to the transient of the sound. Fast attack time “thins” the sound while a slow attack time allows for a “fatter” sound. Set to your taste. STEP 2 – SETTING YOUR RELEASE TIME ▸ Attack Time is committed; Ratio at maximum; Threshold at low ▸ Tune your ears to: Listen to the tempo and nd the “groove” for the release time. Too short may create unwanted pumping and breathing while too slow may compress the next note. fi UNLOCKING YOUR COMPRESSOR STEP 3 – SETTING YOUR RATIO ▸ Attack time committed; Release time committed ▸ Threshold at low ▸ Tune your ears to: Listen for a more subtle effect than before. Think size versus control of the sound. A higher ratio = more control with a smaller sound. A lower ratio = less control with a bigger sound. STEP 4 – SETTING YOUR THRESHOLD ▸ Attack Time committed Release Time committed Ratio committed ▸ Tune your ears to: Listen to the changes in compressed versus un-compressed. Compression sounds best moment to moment, not all the time. A sensitive/low threshold = less realistic, more lifeless sound. An average threshold = more realistic, dynamic sound. STEP 5 - ADJUST OUTPUT GAIN ▸ Observe average Gain Reduction and use the compressor output/makeup gain to compensate. COMPRESSOR TIPS ORDER OF OPERATIONS ▸ Always set input trim prior to inserting a compressor to ensure proper gain structure into the device. IS IT BETTER? ▸ Bypass often while setting to ensure results are an improvement. COMPRESSION DOESN’T INCREASE LOUDNESS, IT INCREASES CONSISTENCY OF SIGNAL ▸ Ensure make-up gain is used to compensate for gain reduction, not simply as a means of increasing the loudness a signal - the job of the fader - this will keep gain structure in check. TO SQUASH OR NOT TO SQUASH? ▸ Unless using compression in parallel, ensure gain reduction isn’t necessarily constant. This will avoid “sucking the life” out of a sound. ▸ Generally speaking, lower ratios when tracking should avoid over-compression (up to 4:1). PRACTICAL NOISE GATE AND EXPANDER APPLICATION SETTING YOUR EXPANDER/GATE ▸ Zero settings: Threshold = 0dB; Attack: fast; Release: fast; Hold: fast; Range: high; Ratio: high or “Gate” setting. ▸ Lower threshold until transients of the desired sound become audible, other sounds ideally do not open the gate. ▸ Increase hold time until length of the desired note is achieved. ▸ Increase release to adjust how quickly following the hold time the expander returns to its fully gain- reduced range. ▸ To increase natural-ness, lower ratio and reduce range to attenuate, rather than mute, the undesired content. PRACTICAL NOISE GATE AND EXPANDER APPLICATION EXPANDER TIPS ▸ If a sound has a sharp transient affected by even the fastest attack settings, use look-ahead values. ▸ If a unwanted leakage is opening the gate, try ltering tonal content so only desired frequencies open with key input. ▸ If a gated sound is “chattering” in its decay, try increasing hysteresis - a second, lower threshold that helps smoothen decay. fi  © Copyright MOSMA (Mid-Ocean School of Media Arts), 2024. 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