Audio Controls

Selects the intercarrier audio subcarrier system. The audio carrier frequency is set relative to the picture carrier by a fixed offset that varies by standard.

NTSC-M — 4.5 MHz intercarrier. FM deviation ±25 kHz. 75 μs pre-emphasis (same as FM broadcasting in the Americas). The 4.5 MHz intercarrier tone, audible in the audio channel when colour subcarrier energy leaks, gives NTSC sets the characteristic 920 Hz buzz pitch (the beat between 3.58 MHz subcarrier and the 4.5 MHz intercarrier offset).

PAL B/G — 5.5 MHz intercarrier. FM deviation ±50 kHz. 50 μs pre-emphasis. Beat with PAL 4.43 MHz subcarrier gives a 1.07 MHz intercarrier buzz component, mixed down to audible buzz by AM nonlinearities in the FM detector.

PAL I (UK) — 6.0 MHz intercarrier. The wider offset was chosen because the UK UHF band spacing used a different channel grid.

SECAM / System L — 6.5 MHz audio for System L; same 6.0 MHz for standard SECAM channels. System L used AM picture modulation (positive), making it incompatible with other European standards even though the audio standard is similar.

Interactions: The audio standard is automatically set when you change the TV Standard, but can be overridden. Changing to a mismatched audio standard produces the correct demodulated audio but changes the pitch and character of any intercarrier buzz.

Broadcast reference test tone. Engineering departments transmitted a test tone before programmes to allow receiving stations to set audio levels and check phase. Each broadcasting organisation used slightly different conventions.

BBC (UK) — 400 Hz continuous tone at −18 dBFS (PPM 4, line-up level). Derived from ITU-R BS.645-2, EBU R68-2000 and EBU Tech 3304. The 400 Hz choice was deliberate: it sits in the middle of the speech intelligibility band and is easy to identify aurally. The BBC used this tone from the early television era through the transition to digital.

SMPTE / ANSI (US) — 1000 Hz at −20 dBFS (0 VU on the analogue meter). 47 CFR §11.31 specifies this for EAS audio line-up. Broadcast network affiliates used 1 kHz for inter-facility exchange level calibration.

IEC 268 — 1000 Hz international alignment standard as specified in IEC 60268-17. Used in international exchange of programme material.

NHK (Japan) — 400 Hz, matching NHK’s alignment convention for MUSE Hi-Vision broadcasts and inter-facility feeds.

Interactions: Test tone overrides the live audio from the source. The tone is carried on the intercarrier FM subcarrier, so Intercarrier Buzz will modulate it when colour is present in the picture.

How to test: Select BBC 400 Hz and mute all other audio sources. A clear 400 Hz tone should be audible at a consistent level. Switch to SMPTE 1 kHz and compare the pitch. Enable Intercarrier Buzz with a colour-bar source — a buzz component appears on top of the test tone.

Chroma-to-audio cross-coupling. In all analogue TV receivers that use the intercarrier method (the vast majority of sets made after 1955), the audio FM demodulator sees the combined video+audio intercarrier signal. The colour subcarrier is present in this signal at the beat frequency between the picture carrier and the sound carrier. In a nonlinear FM detector, the chroma subcarrier amplitude-modulates the audio output, producing a buzz tone whose frequency and timbre vary with the colour content of the image.

The buzz frequency on NTSC sets (920 Hz) is the difference between the 4.5 MHz intercarrier and the 3.58 MHz subcarrier, beat-detected. On PAL-B/G (1.07 MHz) it is further mixed down. The result is an audible buzz that changes pitch and intensity as colours change in the picture — loudest on fully saturated pure colours, absent on monochrome content.

Interactions: Only audible on colour sources (colour bars, live camera with colour). Silent on B&W Mode sources. Higher Saturation increases chroma energy and therefore worsens the buzz. The buzz character changes with the TV Standard because the intercarrier frequency changes.

How to test: Enable with a saturated colour-bar source. An audible buzz is heard that changes in character as different colour bars are displayed. Switch to B&W Mode — the buzz disappears because there is no chroma energy to couple into the audio path.

VHS audio recording path. Standard VHS linear audio uses a separate fixed head to record the audio longitudinally along the edge of the tape. This gives a frequency response of approximately 50 Hz–10 kHz and a signal-to-noise ratio of around 43 dB — noticeably worse than FM broadcast and similar to cassette tape quality.

VHS HiFi (introduced 1983) depth-multiplexes the audio FM signal beneath the video heads’ signal on the helical tracks. This gives frequency response of 20 Hz–20 kHz and SNR around 80 dB — better than CD. HiFi audio is available on VHS-SP and VHS-LP but not VHS-EP (the head penetration depth is insufficient at slow speed).

S-VHS, Hi8 and Betacam SP always use the HiFi/component audio path. Betamax (pre-1983) used linear audio only; later Betamax Hi-Fi used the same depth-multiplexing principle.

Interactions: Linear audio has characteristic tape hiss that is audible in quiet passages. HiFi audio has very low noise but can exhibit a distinctive dropout-concealmentclick when tape dropout hits a HiFi audio block (the linear track is used as backup).

FM audio pre-emphasis and de-emphasis. FM broadcasting (and TV audio) applies a high-frequency boost before transmission and an equal cut at the receiver. This raises the high-frequency audio above the FM threshold noise floor, improving the perceived signal-to-noise ratio for programme material with the typical 1/f energy spectrum.

The time constant determines the corner frequency of the shelving filter: 50 μs (corner at 3.18 kHz) is used in Europe and internationally (ITU-R BS.468); 75 μs (corner at 2.12 kHz) is used in the Americas (NTSC-M audio and FM broadcasting). Applying the wrong de-emphasis at the receiver causes high-frequency roll-off (50 μs decoded with 75 μs) or high-frequency boost (75 μs decoded with 50 μs).

How to test: Select 75 μs pre-emphasis with a NTSC source. The audio will have correct high-frequency balance. Switch to 50 μs de-emphasis — high-frequency content (hiss, speech consonants) becomes noticeably brighter, as if the treble were boosted, because the pre-emphasis is no longer matched by the de-emphasis.