There are a variety of different audio connectors available. The most common types are 3-pin XLR, RCA, and 6.5mm jacks (also known as ¼” jacks).
3-pin XLR connectors are mainly used for balanced audio signals. Using a balanced signal reduces the risk of inference.
Pin 1 is the earth (or shield)
Pin 2 is the +ve (or ‘hot’)
Pin 3 is the -ve (or ‘cold).
There are a number of different XLR’s – 3-pin, 4-pin, 5-pin etc
|3-pin XLR Male||*||*|
|3-pin XLR Female||*||*|
¼” Jack (6.5mm Jack)
There are two types of 6.5mm Jacks: Mono and stereo. The mono jack has a tip and a sleeve, the stereo jack has ring, a tip and a sleeve.
On the mono jack the tip is the +ve, and the sleeve is the -ve or shield.
On a stereo jack being used for a balanced signal, the tip is the +ve, the ring is the -ve, and the sleeve is the shield.
On a stereo jack being used for a stereo signal (left and right), the tip is the left, the ring is the right, and the sleeve is the shield.
Jacks also come in various sizes – 6.5mm (¼”), 3.5mm, 2.5mm. The wiring for all of them is the same.
|1/4″ Mono Jack||*||*|
|1/4″ Stereo Jack||*||*|
RCAs are used a lot for home stereos, videos, DVDs etc.
The RCA can carry either audio or video. It is wired the same way as a mono jack: The center pin is the +ve, and the outer ring is the -ve or shield.
Stereo Jack to 2x RCA
When a stereo 1/4″ jack is being used for a stereo signal (as opposed to a balanced mono signal), the left and right parts of the stereo signal can be split off to two seperate connectors. For example, a stereo headphone output can be split into left and right connectors, and one possible use for this would be to use these two independant connectors to feed left and right monitoring speakers.
XLR to 1/4″ Mono Jack
The most comon way to wire a 3-pin XLR to a 1/4 inch mono jack (or 6.5mm jack), is to join the -ve and shield together.
This can be done by either soldering the shield and -ve wires to the sleeve of the jack……
Or by soldering a jumper on the XLR…..
Either way gives you the same result: An unbalanced audio cable.
XLR to 1/4″ TRS Connector (wired for balanced mono)
The usual way to connect a 3-pin XLR to a 1/4″ TRS (AKA stereo jack plug) is to use the following pin allocation:
XLR pin 1 to jack sleeve
XLR pin 2 to jack tip
XLR pin 3 to jack ring
This wiring configuration gives you a balanced mono audio cable.
XLR to 1x RCA
When connecting a 3-pin XLR to one RCA, you use the same wiring as if you were connecting an XLR to a 1/4″ jack.
The -ve and shield of the XLR are joined together, either at the XLR end or the RCA end. The easiest way is to solder a link between pins 1 and 3 (shield and -ve) of the XLR, rather than trying to solder the shield and -ve wire to the sleeve contact of the RCA.
This produces an unbalanced audio cable.
XLR to 2x RCA
A 3-pin XLR with a stereo signal can be split into left and right by wiring pin 2 of the XLR to the tip of one RCA plug, and pin 3 of the XLR to another RCA tip. Pin 1 of the XLR connects to the sleeve of both RCA plugs.
Practicing the audio cabling
Audio file format
It is a container format for storing audio data on a computer system.
The general approach towards storing digital audio is to sample the audio voltage which, on playback, would correspond to a certain position of the membrane in a speaker of the individual channels with a certain resolution — the number of bits per sample — in regular intervals (forming the sample rate). This data can then be stored uncompressed, or compressed to reduce the file size.
Types of formats
It is important to distinguish between a file format and a codec. A codec performs the encoding and decoding of the raw audio data while the data itself is stored in a file with a specific audio file format.
There are three major groups of audio file formats:
Uncompressed audio format
There is one major uncompressed audio format, PCM, which is usually stored as a .wav on Windows or as .aiff on Mac OS. WAV is a flexible file format designed to store more or less any combination of sampling rates or bitrates. This makes it an adequate file format for storing and archiving an original recording. A lossless compressed format would require more processing for the same time recorded, but would be more efficient in terms of space used. WAV, like any other uncompressed format, encodes all sounds, whether they are complex sounds or absolute silence, with the same number of bits per unit of time. As an example, a file containing a minute of playing by a symphonic orchestra would be the same size as a minute of absolute silence if they were both stored in WAV.
Free and open file formats (uncompressed)
- wav – standard audio file container format used mainly in Windows PCs. Commonly used for storing uncompressed (PCM), CD-quality sound files, which means that they can be large in size — around 10 MB per minute. Wave files can also contain data encoded with a variety of codecs to reduce the file size (for example the GSM or mp3 codecs). Wav files use a RIFF structure.
- ogg – a free, open source container format supporting a variety of codecs, the most popular of which is the audio codec Vorbis. Vorbis offers compression similar to MP3 but is less popular.
- aiff – the standard audio file format used by Apple. It is like a wav file for the Mac.
- raw – a raw file can contain audio in any codec but is usually used with PCM audio data. It is rarely used except for technical tests.
Compressed (Proprietary) formats:
- mp3 – the MPEG Layer-3 format is the most popular format for downloading and storing music. By eliminating portions of the audio file that are essentially inaudible, mp3 files are compressed to roughly one-tenth the size of an equivalent PCM file while maintaining good audio quality.
- wma – the popular Windows Media Audio format owned by Microsoft. Designed with Digital Rights Management (DRM) abilities for copy protection.
- ra – a Real Audio format designed for streaming audio over the Internet. The .ra format allows files to be stored in a self-contained fashion on a computer, with all of the audio data contained inside the file itself.
- ram – a text file that contains a link to the Internet address where the Real Audio file is stored. The .ram file contains no audio data itself.
Compretion and Limitation:
- prevent over modulation, and minimize it when it occurs
- maximize overall loudness
Threshold is the level above which the signal is reduced. It is commonly set in dB, where a lower threshold (e.g. -60 dB) means a larger portion of the signal will be treated (compared to a higher threshold of -5 dB).
Different compression ratios
The ratio determines the input/output ratio for signals above the threshold. For example, a 4:1 ratio means that a signal overshooting the threshold by 4 dB will leave the compressor 1 dB above the threshold. The highest ratio of ∞:1 is commonly achieved using a ratio of 60:1, and effectively denotes that any signal above the threshold will be brought down to the threshold level (unless some attack is in force).
Attack and release
A compressor might provide a degree of control over how quickly it acts. The ‘attack phase’ is the period when the compressor is increasing gain reduction to reach the level that is determined by the ratio. The ‘release phase’ is the period when the compressor is decreasing gain reduction to the level determined by the ratio, or, to zero, once the level has fallen below the threshold. The length of each period is determined by the rate of change and the required change gain reduction. For more intuitive operation, a compressor’s attack and release controls are labelled as a unit of time (often milliseconds). This is the amount of time it will take for the gain to change a set amount of dB, decided by the manufacturer, very often 10 dB. For example, if the compressor’s time constants are referenced to 10 dB, and the attack time is set to 1 ms, it will take 1 ms for the gain reduction to rise from 0 dB to 10 dB, and 2 ms to rise from 0 dB to 20 dB
Soft and hard knees
Hard Knee and Soft Knee compression
Another control a compressor might offer is hard/soft knee. This controls whether the bend in the response curve is a sharp angle or has a rounded edge. A soft knee slowly increases the compression ratio as the level increases and eventually reaches the compression ratio set by the user. A soft knee reduces the audible change from uncompressed to compressed, especially for higher ratios where the changeover is more noticeable. 
An audio engineer might use a compressor subtly in order to reduce the dynamic range of source material in order to allow it to be recorded optimally on a medium with a more limited dynamic range than the source signal, or they might use a compressor in order to deliberately change the character of an instrument being processed.
Engineers wishing to achieve dynamic range reduction with few obvious effects might choose a relatively high threshold and low compression ratio so that the source material is being compressed very slightly most of the time. To deliberately soften the attack of a snare drum, they might choose a fast attack time and a moderately fast release time combined with a higher threshold. To accentuate the attack of the snare, they might choose a slower attack time to avoid affecting the initial transient. It is easier to successfully apply these controls if the user has a basic knowledge of musical instrument acoustics.
It should be noted that compression can also be used to lift the soft passages of a selection, pulling the sound toward a compressed “middle”. Hence, loud sounds are pulled back and soft passages are boosted.
Compression and limiting are no different in process, just in degree and in the perceived effect. A limiter is a compressor with a higher ratio, and generally a fast attack time. Most engineers consider a ratio of 10:1 or more as limiting, although there are no set rules. Engineers sometimes refer to soft and hard limiting which are differences of degree. The “harder” a limiter, the higher its ratio and the faster its attack and release times.