Music - studio and stage notes

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This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Analog audio stuff

Voltages and impedances

Many analog audio levels are not so well standardized, some have changed over time, some quietly. So assume each can be a factor of two off and can require a little knob twiddling at least.

phono

phono input (typically on RCA plugs - which are also used for the higher line level)

order of a milliVolt, and seems to often be the not-yet-amplified output of phono cartridges

There are two common types:

Moving Magnet (MM) pickups give ~2.5mV,

Moving Coil (MC) give ~0.2mV

MC versus MM is one of those debates. Higher-end is usually MC, but quality also significantly depends on other factors.

phono pre-amps will amplify this to (typically) consumer line level (and impedance)

phono directly on line-level (or mic level) input as-is will be very too quiet (or if you manage to amplify it, very noisy)

avoid connecting non-phono and in particular line-level outputs to phono inputs, it may be possible to blow the preamp

impedance

cartridge output: varies, order of 500 Ohm or lower (verify)

phono amp imput: 47k Ohm

consumer microphone level

consumer microphone level (mono, often on 3.5mm TS plugs)

on the order of ~10mV signal output (verify)

...because these are typically electret mics, and that's the order of what a 1cm electret (with FET) will output

3.5mm TS mic input will

very usually provide bias voltage for such electrets to function

a moderate amoubnt of amplification to get the weaker level to the same level as line

technically you could do both on one socket with some extra switches, but separate plugs is easier to explain to people

impedance

mic output - high-impedance microphones are typically cheaper, e.g. the common electret mic is often 1-2kOhm but some 10kOhm+ (verify)

PC mic in impedance: 1..10kohm (varied over time and with cards)

consumer line level

consumer line level (often stereo, often on RCA or 3.5mm TRS plugs)

on the order of ~300mV (~310mV RMS, ~440mV peak, 0.9V peak-to-peak)

also sometimes known as -10dBV (mostly in situations that also do +4dBu pro levels)

but has varied somewhat over time.

I've seen amplifiers with a sensitivity of 250mV, older ones with 150mV

Some recent devices are moving to higher voltages - amps may choose to deal with up to 1V, or 2V in the case of DVD, Bluray(verify) (perhaps in imitation of pro line level?)

when the the other end is not aware, you may need to attenuate the output, and/or keep the amplification low, to avoid distortion.

impedance(verify)

line in impedance is often ~100 Ohm(verify). Possibly higher, up to 1 kOhm(verify)

line out impedance is often ~10 kOhm(verify). Possibly higher, up to 1 MOhm(verify)

Keep in mind this is not the same as headphone out.

Professional microphone level

professional microphone level (mono, often on XLR3 plugs)

order of 10mV, but can vary because of varying microphones

Can be ~1mV, can be ~200mV (in theory more but this is atypical)

more varied designs, and possible amplification at the mic, means more variation with design and per use

e.g. dynamic mics are lower, due to simpler design but this also makes them better for louder things

...so you will need that gain knob

impedance:

mic output: most are in the 50..200 Ohm range (and usually higher in that range), with some deviations (see more notes around here)

mic preamp/mixer input: order of 1..2kOhm (impedance bridging setup to them)

Professional line level

professional line level (mono, often balanced and on 6.35mm TRS)

is +4dBV, so ~1.2V typical. See notes anove.

various input stage are designed with headroom, meaning they can deal with a bunch more

Instrument level

instrument level (mono)

has no standard, though is generally in a predictable range, often being somewhere between pro mic and pro line level

...in part because it includes things like guitars (not amplified) and e.g. synths (amplified)

instruments will typically be unbalanced, TS

the 6.35mm inputs on mixers will often be balanced (TRS) because this makes sense to use for device interconnects when using these plugs.

They may also accept unbalanced (TS) (will be marked with something like 'bal or unbal'), to allow for a few other things (see also #Plugs.27s_relation_to_balanced.2Funbalanced.2C_voltage_levels.2C_etc.)

and such interfaces may have a switch between instrument and line - which seems to come down to that instrument may be padded (and for interfaces with combo inputs, this would probably apply only to the TRS and not to the XLR part)

output impedance: (verify)

guitars are sort of their own story, a more specific variant of instrument level

guitar pickup impedance is often quite high (see notes below on pickup impedance), and their levels may be fairly low

so a direct box (a.k.a. DI) to make it XLR is a better and easy way to put it on a longer cable to a mixer

guitar amps expect high impedance from a directly connected pickup (typically closeby)

DI boxes tend to have a thru on the input side (which are two plugs wired together and typically entirely equivalent) so that you can both

the DI (towards the mixer) accept that high-impedance guitar

a stage-side amp (used as a monitor) accept that same high-impedance guitar

...and don't have to choose between the two

a few guitar amps may have a built-in DI

in general there can be good reasons to mic the cab instead of using an output (primarily the speaker's sound)

Other levels

Less standard / more varied:

• headphone level (on 3.5mm TRS)

roughly commercial line level, but less of a standard, and can easily be a little higher.

headphone amps tend to aim to drive at least a few milliAmps into a ~30-60Ohm headphone (verify)

there are ~4Ohm headphones, but you really woudn't plug those into everything (likely to distort)

there are 250Ohm-600Ohm headphones, but these need their own preamp (the idea is that you can design for slightly better THD with less load on the amp)

Car audio tends to be on the order of 2V, sometimes 4V (verify)

(a headphone amp is sometimes a good cheat to connect consumer-level things to this)

consumer speaker wires

not really about voltage - they will be proportional to the amplifier's/speaker/s ability and related to the imedance, but for a rough idea, for ~100W speakers you'll see up to a few dozen volts

impedance

speaker load is often around 8 or 4 Ohm (sometimes 2, sometimes 16)

amplifier output impedance is typically very low, say 0.1 Ohm (this is also why the whole 'match your speaker impedance exactly to your amp impedance' thing is nonsense in a literal sense -- but with lower-impedance speakers you should limit how much you turn up the volume, because the maximum sensible power output happens earlier - and above that you get both distortion (THD increases with load) and risk of damage)

pro speaker wires

not really a thing. Most speakers are connected by one of:

XLR: carrying typical XLR line signals to active speakers

Speakon-connected: already-amplified signal to a passive speaker

6.35mm TS: already-amplified signal to a passive speaker (Sometimes avoided, to avoid smoky mistakes on mixers)

Note these cables are different from TS instrument cables, basically in that instrument cables use a thinner core-and-shield, while TS for speakers should be two beefier cores (and shielding is pretty irrelevant)

Speakon carrying already-amplified signal to a passive speaker - somewhat preffered to the previous because it's a one-purpose plug

See also:

• https://en.wikipedia.org/wiki/Line_level#Nominal_levels

Plugs and practicality, on mixers and instruments

XLR3 is pro mic level, which is always balanced/differential, always mono.

mono, because one signal requires a differential pair (and the third pin is shield, not ground)

if you want to carry stereo over XLR, use two cables (depending on the devices you're interconnecting, this may be easier with two balanced 6.35mm TRS instead).

6.35 mm is two different things:

6.35mm TS is unbalanced, mono, often instruments and often called instrument level. Guitar plugs are the same thing, though there is a difference between levels put out by guitars and instruments)

Effect pedals are typically unbalanced, instrument/guitar level

Tip is signal, Sleeve is ground/shield

unpowered instruments may have rather lower voltage levels - but still close enough to gain without much trouble

powered instruments may be somewhere inbetween(verify)

6.35mm TRS is pro line level

(typically) balanced/differential ~1.2V mono, and mixes may mark this as "balanced".

Tip and Ring is the pair, Sleeve is shield (not shared ground)

this seems mostly used for interconnects

(rarely, and recognizably) an insert Y lead to two TS plugs, to put an effect on a mixer insert socket

(rarely) unbalanced stereo. To see this on a device is an exception, and will be noted

Mixers tend to accept both TRS balanced and TS unbalanced, because it's not very hard to design them that way - and avoids some weird cases.

If they do both on the same socket they usually mark that (e.g. "bal/unbal")

Note that unbalanced inputs are not always isolated, so connecting unbalanced things (other than floating instuments) could create common mode issues.

What happens when you...

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Edge cases

• TS mic to XLR:

impedance difference - this is what a DI box is for

• TS instrument to XLR:

a DI bix helps isolate

• Plugging line level output into instrument level input usually so much stronger that it distorts

there is a very small (but non-zero) chance of some damage

• plugging TRS outputs into battery powered pedals is potentially harmful - to the output

specifically when they are switched on by plugging it in

they do this by actually using a TRS socket, and completing the power circuit by having the TS's sleeve short Ring to sleeve

but when you use an actual TRS plug, it puts 9V between ring and sleeve.

• TRS consumer stereo into TS-only input will lose the right channel

because it shorts ring (right) to ground (sleeve)

• TRS consumer stereo into TRS balanced - depends a little on the case

usually sounds quiet and weird, because it presents left minus right as the signal

• TRS balanced into a unbalanced-only TS input

shorts one of the pair to ground

might work, might not?(verify)

• TS output (often instrument) into balanced-only TRS input

the plug will short between sleeve and ring, i.e. tie one of the signal lines to shield.

which on the 'cheaty' single-ended balanced driver design might be fine{{verify{{,

on a halfway proper one can cause damage

while particularly fancy ones may be protected against this case (but don't assume it)

might introduce common mode from the mixer itself. (verify)

Plugging audio where it's not quite intended

Further edge cases, when considering more plugs

Things that won't work / things to avoid

Semi-sorted

Analog audio voltage levels

See Music_-_studio_and_stage_notes#Voltages_and_impedances

Other notes

On microphone impedance

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

DI

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

Direct box, DI box, DI unit, DI. (people argue over whether it stands for direct input, direct injection, direct induction, or direct interface).

Functionally

Takes a high-impedance, unbalanced signal (often pro line level TS), outputs a low-impedance pro-mic-level balanced signal (often pro mic level XLR).

The input is often specifically a high-impedance, passive instrument, e.g. electric guitars and electric bass(verify).

Bonus implications / other reasons DIs are used:

• XLR's lower impedance makes it easier to do a run a longer cable (or same distance with less noise) than you could with the cable type you typically connect on the DI's input side

only partially true for passive DIs because all of the power comes from the instrument

• DIs make it easy to avoid connecting the TS-side ground

which can help avoid ground loops you might have when you e.g. plug unbalanced TS directly into a mixer.

Technical stuff

On loudness (measurement)

VU meter, Peak Programme Meter

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, or tell me)

A VU meter ('Volume Unit') was historically implemented by a galvanometer used as ammeter, being a simple and almost passive component that could show signal strength.

Studio VU meters are standardized in that 0VU is +4dBu - though with footnotes, and flaws and biases.

• The needle's mass (and the spring it acts against) makes it non-instant, effectively introducing a lowpass and a slow falloff effect.

When you want to know "roughly how loud were things recently, on average", both of these things are arguably a feature

• they had at most basic filtering, which means they forget that we don't hear frequencies equally.

for example, significant bass would make many VU meters spike higher than human ears would hear it.

• There are a few standards around VU meters, but you wouldn't necessarily know whether anything adhered to it

Consumer market VU meters rarely did.

• VU meters also don't tell you about peaks and transients, though, and that is more important for broadcast.

...so Peak Programme Meter (PPM) is essentially a more recent, better-quantified, and more specific-purpose variation on VU meters.

They are defined to do filtering, if physical they do active driving of the needle, fast attack and slower release (presumably in part because of the needle, and in part because that's still useful).

While modern PPMs are the things to use for more accurate peak monitoring, to conform to broadcast standards, better notice clipping, and such, they are still not necessarily better at showing rough perceived loudness.

A PPM will often indicate an

• a typical permitted max (which you can cross, but don't want to cross frequently / intentionally)

e.g. at -10dB of its max. (actually typically -9dB, arbitrary but with reasons)

• alignment level (where you typically want the signal to hang around)

e.g. at -20dB of its max

called alignment level because in broadcast, devices are calibrated to this (using a standard sine signal)

which also makes things more human-sensible if you have various devices in your chain

Peak and hold - PPM (and some other meters) may indicate both a fairly responsive level updated constantly, and one that holds a longer-term maximum.

Depending on implementation it stays for a given time and then disappears , or slowly decays over time.

This is mostly useful to check on the loudness of sudden noises, which after all was the point of PPM.

Further notes

• Quasi-PPM will show peaks less intensely when they are shorter than a few ms, True PPM will show peaks however brief,

This is effectively an integration time thing.

• Digital PPM may be Sample PPM (SPPM ), which uses max(samples)) because that's simple to calculate

Note that this can still be ~1dB below the actual waveform peaks (see true peaks).

• VU amd PPM meters rarely show more than 60dB of range, and broadcast may focus on 30dB (presumably due to more compression).

• PPMs are tied to dBu levels - but there are a number of different standards. ...because of course there are.

As such, VU and PPM only roughly relate to each other (until you know everything is adhering to the same standard).

Unsorted

https://blog.frame.io/2017/08/09/audio-spec-sheet/

https://www.soundonsound.com/sound-advice/q-whats-difference-between-ppm-and-vu-meters