Beginners guide to Praat

Technical guide for mono and stereo signals


[Contents][Technical issues][Mono and stereo signals]

This section provides some notes and advice on the concepts of channels, tracks, mono and stereo signals, mono and stereo signal sources, storing and opening stereo files, and playing back stereo signals.

Read through, or select a topic:

Channels, tracks, and mono and stereo signals

  • A channel is a pathway for passing on information, in this context sound information. Physically, it might, for example, be a tube you speak down, or a wire from a microphone to an earphone, or connections between electronic components inside an amplifier or a computer.
  • A track is a physical home for the contents of a channel when recorded on magnetic tape. There can be as many parallel tracks as technology allows, but for everyday purposes there are 1, 2 or 4. Two tracks can be used for two independent mono signals in one or both playing directions, or a stereo signal in one direction. Four tracks (such as a cassette recorder) are organized to work pairwise for a stereo signal in each direction; a mono signal is recorded on one track (same track as the left stereo channel) or on both simultaneously (depending on the tape recorder or on how the mono signal source is connected to the recorder).
  • A mono sound signal contains no directional information. It need only be emitted from one loudspeaker or earpiece, but you can have as many as you like and the signal is still mono. There may be several loudspeakers along a railway platform and hundreds around an airport, but the signal remains mono. It need only be sent through one channel, but you can send it through as many as you like and it remains mono. You cannot create directional information simply by sending a mono signal down two "stereo" channels, but you can conjure an illusion of direction from a mono signal by panning it from channel to channel by twidling the balance control.
  • A stereo sound signal contains synchronized directional information from the left and right aural fields. Consequently it requires at least two channels, one for the left field and one for the right field. The left channel is fed by a mono microphone pointing at the left field and the right channel by a second mono microphone pointing at the right field (you will also find stereo microphones that have the two directional mono microphones built into one piece). Quadraphonic stereo uses four channels, surround stereo has at least additional channels for anterior and posterior directions apart from left and right. Public and home cinema stereo systems can have even more channels, dividing the sound fields into narrower sectors.
  • The standard desktop computer sound systems have two channels, usually with one-plug two-channel inputs (a 3.5 mm stereo telephone plug, there is a picture further down), separately for microphone and line. Channel separation is maintained throughout. Software reads and writes mono signals from and to what is otherwise the left stereo channel. External hardware connected to the inputs conforms to this standard or does its own thing - mono in right channel, mono in both channels, stereo left and right crossed. Note that headsets combining microphone and earphones are sold for various applications (computers, telephony, stenography, intercom systems etc.) and are manufactured with appropriate wiring and plugs. A headset sold for one application might very well not work properly in a different application.
  • Standard computer sound systems have an electronic circuit board (a "sound card") plugged in somewhere inside the computer. The input and output sockets where you plug in your mcrophone etc. are usually somewhere round the back of the computer.
  • Professional sound systems are often built in an external cabinet that you con put on the desk beside the computer or on a convenient shelf. These usually offer individual connection, selection and control of each sound channel, both input and output.
  • A wide range of adapters will help you sort out the audio equipment and channels, but they can also help you make the muddle worse. Adapters will help you connect plug to plug or socket to socket, resolve unmatching plugs or sockets, branch or merge cables etc.
    • Be careful with the socket side of adapters. They are rarely marked whether they are mono or stereo (except on the packet you have already thrown away or in the mind of the person who sold them).
    • Be careful also with some adapter cables, where the connections might or might not be crossed depending on their purpose. Cables are rarely marked, but the information should be on the packet or in the catalogue when you buy them.
    • Be careful when putting adapters together in order to make some combination. Use as few adapters as possible (there is the risk of making a poor connection, and stray capacitances and inductances can add up to create an unwanted filter or sound source that corrupts your signal). New types of adapter appear in the shops from time time, reducing the need to make fantastic combinations. A solution you could not find last year might be there this year.
    • Be careful about attaching adapters, and especially combinations of adapters, directly to the sockets of a device. Adapters are relatively heavy and can apply undue leverage, causing physical damage to a delicate socket. It is better to put the adapter(s) between two lengths of cable, and plug the cables into the device sockets to minimize the physical strain. Remember that adapters are relatively cheap, but damage to equipment might be very costly.
  • Examples of telephone plugs are given below. They come in various sizes: 6.3 mm (the original still used for professional applications), 3.5 mm (for smaller portable devices, computer sound systems), 2.5 mm (for miniature devices, minidisks etc). But the layout is the same:
  • The Mono recorder acquires the left input channel (standard practice for mono signals). This is at the tip of either type of telephone plug.
  • The Stereo recorder acquires both channels. The left channel is at the tip of a stereo telephone plug, the right channel is the pole adjacent to the tip of a stereo telephone plug.
  • Mono Recordings: signal sources

  • The Mono sound recorder always acquires the left channel from the computer sound system. The Stereo sound recorder acquires both channels, and either or both can be kept as mono signals.
  • There is a wide range of audio equipment that might be connected to your computer, delivering activity in one or two channels that you are attempting to capture as a mono signal.  Here are some examples of some of the numerous devices you might be working with:
  • A mono microphone with a mono telephone plug (this is how they are usually sold). When this is plugged into a stereo socket it should ground the right channel, leaving the left channel connected and active (but check yours to make sure).
  • A mono microphone with a homemade (lab-made) connection that wires the microphone mono cable to the tip of a stereo telephone plug in order to feed the left channel of a stereo input. The right channel is not connected and might be noisy. A better solution is to connect the mono cable to both channels of a stereo plug, distributing the mono signal equally to them both. The same end can be achieved by using appropriate adapters instead of wiring your own plug.
  • A mono microphone intended for use with computers (stand-alone or in a headset) is usually supplied ready wired to both channels of a stereo telephone plug and feeds equally into both. Note that complete headsets (earphones and microphone in one set) are also sold for other applications than computers, and they might be wired according to other standards. These microphones are far from hifi quality and will not capture all the fine spectral detail speech, but that might not matter in front of a noisy computer in a noisy room.
  • A stereo microphone placed exactly in front of the speaker to provide equal information in both channels, allowing either to be used for a mono signal. Note that a miniature stereo microphone pinned to the informant's clothing might not provide a balanced signal so you should choose the best channel from the Stereo sound recorder.
  • A mono or stereo taperecorder: Similar considerations apply. In addition, there are so many types of taperecorder that cannot be foreseen here (reel-to-reel or cassette, mono or stereo, full/half/quarter track recorders etc). Only you know how your signal is stored on the tape, and how it is delivered to and from your taperecorder.
  • Minidisc recorders make stereo recordings and the same considerations apply as for getting mono signals from other stereo sources. There are also some special points about minidisk recorders:
    • If there is an analogue output, it can be connected to the computer line input. The digital signal will be converted to analogue for the line and then digitized again in the computer, and you will proceed as if it were a traditional stereo analogue taperecorder.
    • If your minidisk recorder allows it, you can download and save the compressed digital signal straight into the computer without using Praat, see the instruction manual and software that came with the recorder.
    • These devices work with sound data compression that is not handled by Praat (Atrac for Sony devices, or MP3). Your compressed Atrac or MP3 files will need to be converted to some other format recognised by Praat. Remember that these compression methods are lossy and degrade the recording, see also the section about Atrac and MP3 formats.
  • If in doubt about which channel your signal is in, use the Stereo sound recorder in Praat, rather than the Mono recorder, and choose which channel to save as mono.
  • You might feel there is unique information in the right channel of a stereo signal and you might be tempted to combine the two channels of a stereo signal into one mono signal to save that information from being lost:
    • This will not be necessary if your informant is equally balanced between the left and right fields, since both channels will then have virtually equal content and you can save just one channel without losing anything.
    • You possibly have a stereo signal because many current tape recorders always record both channels simultaneously. Using a mono microphone for field work might seem a better solution, but it might not be a good idea to connect a mono plug straight into the microphone socket of a stereo taperecorder, because a phantom microphone DC power supply might be short-circuited. The professional solution is to use separate microphone amplifiers with independent channel selection, which is fine in a recording studio or concert hall, but not in the field. The alternative is to use a stereo microphone and select the mono signal from one channel  while inputting your recording to the Praat recorder. At this point you will be back in the lab, with whatever resources it offers - studio taperecorders, audio mixers, hifi amplifiers etc.
    • If you are working on radio speech that is broadcast in stereo, check if there is a mono/stereo switch on the radio receiver or on an amplifier.
    • If all else fails, try a stereo-to-mono adapter followed by a mono-to-stereo adapter, which should put equal mono signals in both channels, then you can analyse just one. Remember that adapters are relatively heavy. Use a cable for the last bit to the input socket, to avoid placing a mechanical strain on it.
    • If you have waited until the stereo signal is inside the computer, consider the following pitfalls:
      • Averaging the channels is fine if the signal amplitudes are equal in both channels, but there will be a final loss of amplitude if they are not. If one channel is silent but noisy, you will include that noise in the average. If your really do want to average your channels,  Praat allows you to average stereo signals to a mono signal. Proceed as follows: save your stereo signal as a stereo file from the stereo recorder, and then open that stereo file as a mono object from the Objects window, and the resulting sound object will be the average of both channels.
      • If you are determined to merge the channels, you might find it more convenient to do it outside Praat in a sound editor or mixer, where you can control channel balance and amplitudes. Or simply use adapters on the input cable as suggested above.
      • But make sure you have something to gain from merging the channels into one mono signal. It is simpler to use the Stereo recorder in Praat and take the best channel.

Stereo signal sources

  • The Stereo sound recorder always acquires both channels from the computer sound system and makes them available as both mono and stereo signals. The mono feature is useful if you are not sure which channel contains your mono signal. What the two channels actually contain depends entirely on conditions prevailing at each recording instance. These are in general related to the computer, the type of sound system installed on it, what other equipment is connected to the sound system and how, and the properties of your input signal. Here are some examples:
  • A stereo microphone: Both channels are active and either channel can be saved as a mono file, or both can be saved together as a stereo file; either channel or both can be transferred to the objects list as independent mono sound objects.
  • A stereo taperecorder: In principle this will be like using a stereo microphone, except it is connected to the line input. But if the tape being played happens to be mono, the signal may come in on either chanel or both, depending on the design of the tape recorder, and how the signal was originally recorded; you will need to watch the sound recorder level meters and act accordingly.
  • A mono microphone: This might give a signal to one channel (while the other remains silent or noisy) or to both, depending on how the microphone is connected. You should use the Stereo recorder for this if you are not sure which channel the mono signal is in, and watch the signal level meters.
  • A mono taperecorder: Professional mono taperecorders are not so common these days, but someone might be using a studio recorder in a lab. This will be like recording from a mono microphone (see previous item), except it is connected to the line input.
  • A DAT recorder: The standard solution is to use an analogue output and proceed as if it were a traditional analogue tape recorder. This means the DAT recorder converts the digital signal to analogue before sending it, and this signal is then digitized again by the computer sound system, with some consequent quality loss. Alternatively, there are special systems available for transferring the original digital signal directly to the computer, avoiding these extra DA/AD conversions, but then you would not use Praat (and the DAT recorder must have a digital output). Such systems might be expensive, but there is no loss of quality along the way.
  • An external audio CD player: This will usually be attached to the line input like an analogue tape recorder. Here also the digital signal will be converted to analogue for the line and then digitized again in the computer, and you will proceed as if it were a traditional stereo analogue taperecorder.
  • Audio CD disk in a computer CD-ROM drive:
    • If you have a CD-ROM drive that can play audio CD discs, you can record from that. The CD input is selected from the computer sound system, and the stereo signal comes up in the Stereo sound recorder in Praat (here again there is a double DA/AD conversion).
    • An alternative method is to use a program that "grabs" (or "rips") the digital signal from the audio CD and saves it directly as a sound file, in which case you would not use Praat. You might experience crackling sounds, because the programs that grab the audio are often not very good at finding precise locations on audio disks. (For the same reason, if you want to archive your sound data on CDs, save the sound files to a CD-ROM disc rather than make an audio CD for sound backup.)
  • Minidisc recorders etc: There are two ways of getting your stereo signal into the computer:
    • If there is an analogue output, it can be connected to the computer line input. Here also the digital signal will be converted to analogue for the line and then digitized again in the computer, using the Praat Stereo Recorder.
    • It might be possible to download and save the compressed digital signal directly from the recorder into the computer without using Praat, see the instruction manual for the recorder.
    • These devices work with lossy sound data compression that is not handled by Praat (Atrac for Sony devices, or MP3). Your compressed Atrac or MP3 files will need to be converted to some other format that can be recognised by Praat. Note these methods of compression are lossy and the sound is degraded. See the Atrac and MP3 formats.

Opening stereo files in Praat

  • Sound files on disk (the hard disk, or floppies or CD-ROMs etc, but not CD audio disks which have to be ripped by a special program) are opened from the Read menu in the Objects window as Sound objects or as Long sound objects. Sound objects are always mono. Long sound objects are loaded as they are, either mono or stereo. Stereo files are opened in several different ways by Praat, but and there are only a few manipulative operations available in Praat for stereo signals.
    • Read/Read from file: Stereo channels are combined by averaging into one mono Sound object, that can be analysed and edited, and the object name is the same as the original file. This name can be changed in the Objects window. Averaging will lose some amplitude (a moment's reflection will tell you that the average of e.g. 6 and 4 is 5, and in the worst case the average of 6 and 0 is 3). There is a warning that the channels will be combined, and there is no way to refuse. If this is not what you wish to happen you can immediately remove this new sound object from the objects list, and do something else. For example, you can use the next option instead and work on one channel only.
    • Read two sounds from stereo file: Each channel becomes a separate mono Sound object, named left and right respectively. These names can be changed in the Objects list. Either can be analysed or edited independently of the other, but remember this might compromise them as stereo channels (this is only important if you intend to use them together again as a stereo signal). Remember that working on sound objects in the objects list does not affect the original data stored in the sound file on the disk, at least so long as you do not try try to save any manipulations back into the original file.
    • Open long sound file: The resulting Long sound object is the same as the original file, mono or stereo as the case may be. Stereo Long sound objects are played back in stereo. Both channels are displayed if a stereo long sound object is viewed in the Sound editor.
  • Atrac and MP3 files: Praat does not handle these lossy compressed formats, so they will need to be converted by some special program. Note that these compression methods degrade the signal. See the Atrac and MP3 formats.

Saving stereo files in Praat

  • Praat sound objects are usually saved as mono files. However, stereo signals can be saved as stereo files in various ways and from various places in Praat
    • From the Write menu in the Objects window: A selected long sound object that is stereo will be saved as a stereo file. Note that there is nothing in the Objects list to remind you the object is stereo. The Write menu does not offer to save in stereo, but the signal will be saved as stereo (long sound objects are saved as they are)
    • From the Write menu in the Objects window: Two selected Sound objects that constitute a stereo pair can be saved as either a mono file or a stereo file:
      • Write to ... file: The resulting mono sound file will contain the contents of the first object followed by the second. A stereo pair is simply a special case of a general Praat principle here: several selected sound objects are always accepted and sequenced one after the other in one mono file.
      • Write to ... stereo file: The stereo file will contain two channels with one Sound object in one and the other Sound object in the other. If the two objects were a true stereo pair the result is a true stereo file you can listen to in stereo.
      • Note, Praat does not check that they are a true stereo pair and will save any two Sound objects in two channels, filling out any time difference with silence. Only you will know what the two Sound objects really contain. They might be synchronized measurement data, they might be the synchronized channels of a dichotic test. They might be a stereo pair. Or they might be unrelated mono signals. You can listen to such a file and hear one signal in one ear and one in the other.
    • File menu in the Stereo sound recorder: A new stereo recording can be saved as a stereo file directly from the Stereo sound recorder (either channel or both can also be saved individually as mono files).
    • File menu in the Sound editor: If you are viewing a long sound object that is stereo, both channels will be displayed. A selection made by clicking and dragging over the waveform can be saved as a stereo extract.

    Stereo playback

  • To hear a stereo signal in stereo:
    • In the Stereo sound recorder: The Stereo sound recorder always accepts both channels from the computer's sound system. The Play button (Windows, not Sun) will always play back the latest recording in stereo. If the signal happens to be mono (e.g. from a mono microphone, mono tape recording or mono radio program etc), you will hear the mono signal in one ear and silence or noise in the other ear, or you will hear it in both ears if it was connected to both channels.
    • In the Objects window: The only possible stereo signal in the objects list is a long sound object that happens to be stereo. Select a stereo long sound object in the objects list click Play, and you will hear it in stereo
      • It is also possible to add the two channels of a stereo signal to the Objects list as two independent mono Sound objects (from the stereo sound recorder or when opening a stereo file from the disk). There is no way to listen to these in stereo immediately (selected sound objects are played back in sequence).
    • If you have the original stereo file, load it as a Long sound object and listen to that.
    • If you do not have a stereo file, save the two mono Sound objects together as a stereo file (see above) and then open it as a Long sound and listen to that (the result will only be true stereo if they were a genuine stereo pair, otherwise you will hear something different in each ear).
  • In the Sound editor: Both channels of a stereo long sound are displayed when it is viewed in the Sound editor, and both channels are heard in stereo when played back from the Sound editor window.
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    [Contents][Technical issues][Mono and stereo signals]

    Praat for beginners - Sidney Wood - 26 Sep 2004

     Copyright (c) 1995-2004 Sidney Wood. All rights reserved.