key tone

DTMF (Dual-Tone Multi-Frequency)

Click the buttons on the keypad to hear the DTMF tones (Flash 7).

DTMF, better known as touch-tone, is a system of signal tones used in telecommunications. Applications include voice mail, help desks, telephone banking, etc.
There are twelve DTMF signals, each of which is made up of two tones from the following selection: 697 Hz, 770 Hz, 852 Hz, 941 Hz, 1209 Hz, 1336 Hz, 1477Hz.
The tones are divided into two groups (low and high), and each DTMF signal uses one from each group. This prevents any harmonics from being misinterpreted as part of the signal.
The following table shows the frequencies used for each signal:

	1209 Hz	1336 Hz	1477 Hz
697 Hz	1	2	3
770 Hz	4	5	6
852 Hz	7	8	9
941 Hz	*	0	#

Download

The 12 DTMF tones are available for download in the following zip file (right-click and Save As): dtmf.zip
You can download the Flash touch-tone keypad here.

Sound Mixers: Outputs

The main output from most mixing devices is a stereo output, using two output sockets which should be fairly obvious and easy to locate. The connectors are usually 3-pin XLRs on larger consoles, but can also be 6.5mm TR (jack) sockets or RCA sockets.
The level of the output signal is monitored on the mixer's VU meters. The ideal is for the level to peak at around 0dB or just below. However you should note that the dB scale is relative and 0dB on one mixer may not be the same as 0dB on another mixer or audio device. For this reason it is important to understand how each device in the audio chain is referenced, otherwise you may find that your output signal is unexpectedly high or low when it reaches the next point in the chain.
In professional circles, the nominal level of 0dB is considered to be +4 dBu. Consumer-level equipment tends to use -10 dBV.
The best way to check the levels of different equipment is to use audio test tone. Send 0dB tone from the desk and measure it at the next point in the chain.

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Many mixers include a number of additional outputs, for example:
Monitor Feed: A dedicated monitor feed which can be adjusted independently of the master output.
Headphones: The headphone output may be the same as the monitor feed, or you may be able to select separate sources to listen to.
Auxiliary Sends: The output(s) of the mixer's auxiliary channels.
Subgroup Outputs: Some consoles have the option to output each subgroup independently.
Communication Channels: Some consoles have additional output channels available for communicating with the stage, recording booths, etc.

Sound Mixers: Subgroups

Subgroups are a way to "pre-mix" a number of channels on a sound console before sending them to the master output mix. In the following diagram, channels 1 and 2 are assigned directly to the master output bus. Channels 3,4,5 and 6 are assigned to subgroup 1, which in turn is assigned to the master output.

Subgroups have many uses and advantages, the most obvious being that you can pre-mix (sub-mix) groups of inputs.
For example, if you have six backing vocalists you can set up a good mix just for them, balancing each voice to get a nice overall effect. If you then send all six channels to one subgroup, you can adjust all backing vocals with a single subgroup slider while still maintaining the balance between the individual voices.
Note that if your mixing console's subgroups are mono, you will need to use them in pairs to maintain a stereo effect. For each pair, one subgroup is the left channel and the other is right. Each channel can be panned across the two subgroups, while the subgroups are panned completely left and right into the master output bus.

Sound Mixers: Channel Faders

Each channel has it's own fader (slider) to adjust the volume of the channel's signal before it is sent to the next stage (subgroup or master mix).
A slider is a potentiometer, or variable resistor. This is a simple control which varies the amount of resistance and therefore the signal level. If you are able to look into the inside of your console you will see exactly how simple a fader is.
As a rule it is desirable to run the fader around the 0dB mark for optimum sound quality, although this will obviously vary a lot.
Remember that there are two ways to adjust a channel's level: The input gain and the output fader. Make sure the input gain provides a strong signal level to the channel without clipping and leave it at that level — use the fader for ongoing adjustments.

Sound Mixers: PFL

PFL means Pre-Fade Listen. It's function is to do exactly that — listen to the channel's audio at a point before the fader takes effect. The PFL button is usually located just above the channel fader. In the example on the right, it's the red button (the red LED lights when PFL is engaged).
Note: PFL is often pronounced "piffel".
When you press the PFL button, the main monitor output will stop monitoring anything else and the only audio will be the selected PFL channel(s). This does not affect the main output mix — just the sound you hear on the monitor bus. Note that all selected PFL channels will be monitored, so you can press as many PFL buttons as you like.
PFL also takes over the mixer's VU meters.
PFL is useful when setting the initial input gain of a channel, as it reflects the pre-fade level.

PFL vs Solo

PFL is similar to the solo button. There are two differences:

1. PFL is pre-fader, solo is post-fader (i.e. the fader affects the solo level).
2. PFL does not affect the master output but soloing a channel may do so (depending on the mixer).

Sound Mixers: Auxiliary Channels

Most sound desks include one or more auxiliary channels (often referred to as aux channels for short). This feature allows you to send a secondary feed of an input channel's audio signal to another destination, independent of the channel's main output.
The example below shows a four-channel mixer, with the main signal paths shown in green. Each input channel includes an auxiliary channel control knob — this adjusts the level of the signal sent to the auxiliary output (shown in blue). The auxiliary output is the sum of the signals sent from each channel. If a particular channel's auxiliary knob is turned right down, that channel is not contributing to the auxiliary channel.

 

In the example above, the auxiliary output is sent to a monitoring system. This enables a monitor feed which is different to the main output, which can be very useful. There are many other applications for auxiliary channels, including:

• Multiple separate monitor feeds.
• Private communication, e.g. between the sound desk and the stage.
• Incorporating effects.
• Recording different mixes.

Mixers are not limited to a single auxiliary channel, in fact it is common to have up to four or more. The following example has two auxiliary channels — "Aux 1" is used for a monitor and "Aux 2" is used for an effects unit.

Note that the monitor channel (Aux 1) is "one way", i.e. the channel is sent away from the mixer and doesn't come back. However the Aux 2 channel leaves the mixer via the aux send output, goes through the effects unit, then comes back into the mixer via the aux return input. It is then mixed into the master stereo bus.

Pre / Post Fader

The auxiliary output from each channel can be either pre-fader or post-fader.
A pre-fader output is independent of the channel fader, i.e. the auxiliary output stays the same level whatever the fader is set to.
A post-fader output is dependent on the fader level. If you turn the fader down the auxiliary output goes down as well.
Many mixers allow you to choose which method to use with a selector button. The example pictured right shows a mixer channel with four auxiliary channels and two pre/post selectors. Each selector applies to the two channels above it, so for example, the button in the middle makes both Aux 1 and Aux 2 either pre-fader or post-fader.

Sound Mixers: Channel Assigning & Panning One of the last sets of controls on each channel, usually just before the fader, is the channel assign and pan. Pan Control Pan Almost all stereo mixers allow you to assign the amount of panning. This is a knob which goes from full left to full right. This is where the channel signal appears on the master mix (or across two subgroups if this is how the channel is assigned). If the knob is turned fully left, the channel audio will only come through the left speaker in the final mix. Turn the knob right to place the channel on the right side of the mix. Assign This option may be absent on smaller mixers but is quite important on large consoles. The assign buttons determine where the channel signal is sent. In many situations the signal is simply sent to the main master output. In small mixers with no assign controls this happens automatically. However you may not want a channel to be fed directly into the main mix. The most common alternative is to send the channel to a subgroup first. For example, you could send all the drum microphones to their own dedicated subgroup which is then sent to the main mix. This way, you can adjust the overall level of all the drums by adjusting the subgroup level. Channel Assign Buttons and Pan ControlIn the example pictured right, the options are: * Mix: The channel goes straight to the main stereo mix * 1-2: The channel goes to subgroup 1 and/or 2. If the pan control is set fully left the channel goes only to subgroup 1, if the pan is set fully right the channel goes only to subgroup 2. If the pan is centered the channel goes to subgroups 1 and 2 equally. * 3-4: The channel goes to subgroups 3 and/or 4, with the same conditions as above. For stereo applications it is common to use subgroups in pairs to maintain stereo separation. For example, it is preferable to use two subgroups for the drums so you can pan the toms and cymbals from left to right. You can assign the channel to any combination of the available options. In some cases you may not want the channel to go to the main mix at all. For example, you may have a channel set up for communicating with the stage via an aux channel. In this case you don't assign the channel anywhere.

Sound Mixers: Channel Assigning & Panning

One of the last sets of controls on each channel, usually just before the fader, is the channel assign and pan.

Pan

Almost all stereo mixers allow you to assign the amount of panning. This is a knob which goes from full left to full right. This is where the channel signal appears on the master mix (or across two subgroups if this is how the channel is assigned). If the knob is turned fully left, the channel audio will only come through the left speaker in the final mix. Turn the knob right to place the channel on the right side of the mix.

Assign

This option may be absent on smaller mixers but is quite important on large consoles. The assign buttons determine where the channel signal is sent.
In many situations the signal is simply sent to the main master output. In small mixers with no assign controls this happens automatically.
However you may not want a channel to be fed directly into the main mix. The most common alternative is to send the channel to a subgroup first. For example, you could send all the drum microphones to their own dedicated subgroup which is then sent to the main mix. This way, you can adjust the overall level of all the drums by adjusting the subgroup level.
In the example pictured right, the options are:

• Mix: The channel goes straight to the main stereo mix
• 1-2: The channel goes to subgroup 1 and/or 2. If the pan control is set fully left the channel goes only to subgroup 1, if the pan is set fully right the channel goes only to subgroup 2. If the pan is centered the channel goes to subgroups 1 and 2 equally.
• 3-4: The channel goes to subgroups 3 and/or 4, with the same conditions as above.

For stereo applications it is common to use subgroups in pairs to maintain stereo separation. For example, it is preferable to use two subgroups for the drums so you can pan the toms and cymbals from left to right.
You can assign the channel to any combination of the available options.
In some cases you may not want the channel to go to the main mix at all. For example, you may have a channel set up for communicating with the stage via an aux channel. In this case you don't assign the channel anywhere.

Sound Mixers: Channel Equalization

Most mixers have some of sort equalization controls for each channel. Channel equalizers use knobs (rather than sliders), and can be anything from simple tone controls to multiple parametric controls.
Note: For more general information about equalization see Audio Equalizers.
The first example on the right is a simple 2-way equalizer, sometimes referred to as bass/treble or low/high. The upper knob adjusts high frequencies (treble) and the lower knob adjusts low frequencies (bass). This is a fairly coarse type of equalization, suitable for making rough adjustments to the overall tone but is not much use for fine control.

 

This next example is a 4-way equalizer. The top and bottom knobs are simple high and low frequency adjustments (HF and LF).
The middle controls consist of two pairs of knobs. These pairs are parametric equalizers — each pair works together to adjust a frequency range chosen by the operator. The brown knob selects the frequency range to adjust and the green knob makes the adjustment.
The top pair works in the high-mid frequency range (0.6KHz to 10KHz), the lower pair works in the low-mid range (0.15 to 2.4KHz).
The "EQ" button below the controls turns the equalization on and off for this channel. This lets you easily compare the treated and untreated sound.
It is common for mixers with parametric equalizers to combine each pair of knobs into a single 2-stage knob with one on top of the other. This saves space which is always a bonus for mixing consoles.

Notes About Channel Equalization

If the mixer provides good parametric equalization you will usually find that these controls are more than adequate for equalizing individual sources. If the mixer is limited to very simple equalization, you may want to use external equalizers. For example, you could add a graphic equalizer to a channel using the insert feature.
In many situations you will use additional equalization outside the mixer. In live sound situations, for example, you will probably have at least one stereo graphic equalizer on the master output.