Audio Connectors and Signal Formats

You can use a variety of analog and digital audio connection options and signal formats.

Analog Audio Connectors and Signal Formats

Different audio connectors are suited for different purposes. Audio connectors are often indicative of the kind of signal they transmit. However, there are enough exceptions that it’s important to know what kind of audio signal you are connecting, in addition to the connector type. An important distinction is whether an audio connector carries a balanced or an unbalanced signal.

1/8" Mini Connectors

These are very small, unbalanced audio connectors. Many computers have 1/8" mini inputs and outputs at –10 dBV line level, and many portable audio devices such as CD players, iPod digital music players, and MP3 players use these connectors for headphone outputs. Portable MiniDisc and DAT recorders often use 1/8" mini connectors for connecting microphones.

Figure. Diagram showing a mono miniplug connector and a stereo miniplug connector.

Note: Some Mac computers and portable audio recorders also use a connector that combines both a stereo miniplug and a 1/8" optical digital connection (see S/PDIF) in a single jack.

RCA Connectors

Most consumer equipment uses RCA connectors, which are unbalanced connectors that usually handle -10 dBV (consumer) line levels.

Figure. Diagram showing an RCA connector.

1/4" Tip-Sleeve Connectors

1/4" tip-sleeve (TS) connectors (also called 1/4" phone connectors) with a tip and a sleeve are unbalanced connectors often used for musical instruments like electric guitars, keyboards, amplifiers, and so on.

Figure. Diagram showing a 1/4-inch tip-sleeve connector.

1/4" Tip-Ring-Sleeve Connectors

Professional equipment often uses 1/4" tip-ring-sleeve (TRS) audio connectors with +4 dBu line level. 1/4" TRS connectors connect to three wires in an audio cable—hot, neutral, and ground—and usually carry a balanced audio signal. In some situations, the three wires may be used to send left and right (stereo) signals, making the signals unbalanced.

Figure. Diagram showing a 1/4-inch tip-ring-sleeve connector.

Note: Tip-sleeve and tip-ring-sleeve connectors look almost identical. Some audio devices (especially mixers) accept a TS connector in a TRS jack, but you should always check the equipment documentation to be sure. Remember that most 1/4" TS connectors connect to –10 dBV line level equipment, whereas 1/4" TRS connectors usually expect a +4 dBu line level.

XLR Connectors

These are the most common professional audio connectors. They almost always carry a balanced signal. Many cables use an XLR connector on one end and a 1/4" TRS connector on the other. The signal may be microphone level (when using a microphone) or +4 dBu/dBm (professional) line level.

Figure. Diagram showing an XLR connector.

Digital Audio Connectors and Signal Formats

Although digital audio signals are completely different from analog audio signals, the same connectors are often used for convenience. For example, an XLR connector can be used to carry an analog audio signal or an AES/EBU digital audio signal.

AES/EBU

The AES/EBU digital audio specification was jointly developed by the Audio Engineering Society (AES) and the European Broadcasting Union (EBU). AES/EBU audio signals typically use XLR connectors, but 25- or 50-pin D-subminiature connectors can also be used for multiple channels on interfaces or mixers.

Traditionally, AES/EBU sample rates were limited to 44.1 and 48 kHz at up to 24 bits per sample. However, a “dual wire” mode allows some equipment to pair AES/EBU connectors to increase the sample rate. Some newer devices also support “single wire” mode with sample rates up to 192 kHz.

S/PDIF

Sony/Philips Digital Interface Format (S/PDIF) is a consumer-level variation of the AES/EBU digital audio protocol. S/PDIF audio data can be transmitted several ways, including:

  • Via coaxial cables with an RCA connector

  • Via optical TOSLINK

Connectors for S/PDIF signals are found on most consumer digital equipment, such as DAT recorders, CD players, DVD players, MiniDisc equipment, and some audio interfaces.

Dolby Digital (AC-3)

Dolby Digital (AC-3) is a compressed digital audio signal format used for transmission of 5.1-channel surround sound. Mono and stereo signals can also be carried in this format. Typically, this audio signal is embedded within a S/PDIF signal and carried via TOSLINK or coaxial cables with RCA connectors.

DTS

Digital Theater System (DTS) is a compressed digital audio signal format used for transmission of 5.1-channel surround sound. This format is primarily used in movie theaters and on DVD releases. More recent variations of DTS support more than six channels of audio. This audio signal is usually embedded within a S/PDIF signal and carried via TOSLINK or coaxial cables with RCA connectors.

TOSLINK

TOSLINK is an optical digital audio format developed by the Toshiba Corporation. These digital audio formats can be transmitted via TOSLINK optical cables and connectors:

  • S/PDIF

  • ADAT Lightpipe

Some Mac computers have a single interface that combines a TOSLINK connector with an analog stereo miniplug.

Figure. Diagram showing a TOSLINK connector.

ADAT Lightpipe

ADAT Lightpipe is an eight-channel digital audio format developed by Alesis. This signal format uses TOSLINK optical connectors. Eight channels are supported at sample rates of 44.1 and 48 kHz using 24 bits per sample. Higher sample rates are available by pairing channels. (This format is sometimes called sample multiplexing, or S/MUX.) For example, a sample rate of 192 kHz is possible, but the number of channels is reduced to two. However, not all equipment supports channel pairing and increased sample rates.

TDIF

Tascam Digital Interface (TDIF) is a signal format for transferring digital audio between Tascam digital multitrack recorders or digital mixers. A 25-pin D-subminiature connector is used. Eight channels are supported at sample rates of 44.1 and 48 kHz using 24 bits per sample. Higher sample rates are available by pairing channels.

Figure. Diagram showing a TDIF connector.

About Analog Audio Levels

There are six basic kinds of analog audio levels found on most equipment:

  • Microphone level: Around 50 or 60 dB less than line level. When you use a microphone, the level is very low, requiring a preamplifier to raise the signal to line level before it can be recorded or processed. Most audio mixers, cameras, and professional portable recording devices have built-in preamplifiers.
  • Instrument level: Between microphone and line level, around –20 dBV or so. Guitars and keyboards usually output at instrument level.
  • Line level (consumer): Consumer line level is output at –10 dBV.
  • Line level (professional): Professional line level is output at +4 dBu (or dBm in older equipment).
  • Speaker level: This signal varies considerably depending on the amplifier used, but it is very strong compared to the others because it is used to drive speakers.
  • Headphone level: This signal is like speaker level, but much lower. The sole purpose of this signal is to drive stereo headphones.

About Units of Analog Audio Measurement

Professional audio equipment typically uses higher voltage levels than consumer equipment, and it also measures audio on a different scale. Keep the following points in mind when using consumer and professional audio equipment together:

  • Professional analog devices measure audio using dBu (or dBm in older equipment). 0 dB on the audio meter is usually set to +4 dBu, which means optimal levels are 4 dB greater than 0 dBu (.775 V), or 1.23 V.

  • Consumer audio equipment measures audio using dBV. The optimal recording level on a consumer device is –10 dBV, which means the levels are 10 dB less than 0 dBV (1 V), or 0.316 V.

Therefore, the difference between an optimal professional level (+4 dBu) and consumer level (–10 dBV) is not 14 dB, because they are using different signals. This is not necessarily a problem, but you need to be aware of these level differences when connecting consumer and professional audio equipment together.

About Balanced Audio Signals

Audio cables can be either balanced or unbalanced, depending on their intended use. For long cable runs, especially when using relatively low microphone levels, a three-wire balanced audio circuit reduces noise. Balanced audio cables use the principle of phase cancelation to eliminate noise while maintaining the original audio signal.

A balanced audio cable sends the same audio signal on two wires, but inverts the phase of one signal by 180 degrees.

Figure. Diagram showing an audio signal and an inverted signal.

When noise is introduced into the cable, it is introduced equally to both the original and the inverted signal.

Figure. Diagram showing an audio signal with noise.

When the signal arrives at its destination, the inverted signal is put back in phase and both signals are combined. This puts the original and inverted signals back in phase, but it causes the noise signals on each line to be out of phase.

Figure. Diagram showing an audio signal and an inverted audio signal.

Now, both audio signals are in phase, but the noise is inverted, causing the noise to be canceled. At the same time, the original signal gets a little stronger because it is sent on two wires and combined. This helps compensate for the reduction in signal strength that occurs naturally on a long cable run.

Figure. Diagram of a combined audio signal witih noise eliminated.

Any noise introduced into the cable across its long run is almost completely eliminated by this process.

Note: Unbalanced cables have no way of eliminating noise and are therefore not as robust for long-distance cable runs, microphone signals, and other professional uses.