Physics:Multichannel television sound

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Multichannel television sound, better known as MTS (often still as BTSC, for the Broadcast Television Systems Committee that created it), is the method of encoding three additional channels of audio into an analog NTSC-format audio carrier.


Multichannel Television Sound was adopted by the Federal Communications Commission as the United States standard for stereo television transmission in 1984. Initial work on design and testing of a stereophonic audio system began in 1975 when Telesonics approached Chicago public television station WTTW. WTTW was producing a music show titled Soundstage at that time, and was simulcasting the stereo audio mix on local FM stations. Telesonics and WTTW formed a working relationship and began developing the system which was similar to FM stereo modulation. Twelve WTTW studio and transmitter engineers added the needed broadcast experience to the relationship. The Telesonics system was tested and refined using the WTTW transmitter facilities on the Sears Tower. In 1979, WTTW had installed a stereo Grass Valley master control switcher, and had added a second audio channel to the microwave STL (Studio Transmitter Link). By that time, WTTW engineers had further developed stereo audio on videotape recorders in their plant, using split audio track heads manufactured to their specifications, outboard record electronics, and Dolby noise reduction that allowed Soundstage to be recorded and electronically edited. In addition, an Ampex MM1100, 24-track audio recorder was also used for music production and mixing. PBS member stations who wished to deliver Soundstage in stereo were provided with a four-track (left, right, vertical drive, and time code) audio tape that could be synced with the video machines in those cities.

During the FCC approval process, several manufacturers applied to the FCC for consideration. Most notably the Electronic Industries Alliance (EIA) and Japanese EIA asked to be included in order to represent their members in the testing and specification phases of the approval process. WTTW engineers helped set standards for frequency response, separation, and other uses of the spectrum. They also provided program source material used for the testing and maintained the broadcast chain. A 3M 24-track audio recorder was used to allow the selection of 12 different stereo programs for testing. The Matsushita Quasar TV manufacturing plant and laboratory, just west of Chicago, was used as the source for all testing of the competing systems. Following the testing, several questions were raised about the validity of some of the tests, and a second round of testing began.

WTTW installed a Broadcast Electronics prototype stereo modulator in October 1983, and began full-time broadcasting in stereo at that time using the Telesonics system prior to FCC rule-making on the BTSC system. Following EIA and FCC recommendations, the BE modulator was modified to meet BTSC specifications, and by August 1984 was in full-time use on WTTW.

Sporadic network transmission of stereo audio began on NBC on July 26, 1984, with The Tonight Show Starring Johnny Carson, although at the time only the network's New York City flagship station, WNBC, had stereo broadcast capability;[1] regular stereo transmission of NBC programs began during early 1985. ABC and CBS followed suit in 1986 and 1987, respectively. FOX was the last network to join around 1990, with the four networks having their entire prime-time schedules in stereo by late 1994 (The WB and UPN launched the following season with their entire line-ups in stereo). One of the first television receiving systems to include BTSC capability was the RCA Dimensia, released in 1984.[2]

From the mid-1980s to the mid-1990s and also in the 2000s, the networks would display the disclaimer "In stereo (where available)" at the beginning of stereo programming.

Adopted in

It has also been adopted by

How MTS works

The first channel is the stereo difference (left minus right), used to add stereophonic sound to the existing monophonic (the left plus right stereo sum) audio track.

In other words, the normal mono television audio consists of L+R information. A second signal (MTS) rides on top of this mono carrier wave. This MTS signal consists of L minus R.

  • When the two audio channels are added together, or summed (L+R plus L-R), the left channel is derived.
  • When the second audio channel is subtracted from the first by a phase reversal (L+R minus L-R), the right channel is derived.

MTS real world performance

  • In ideal circumstances MTS Stereo is about 1.5 db better in performance than standard VHF FM stereo.[citation needed]
  • Usually with MTS, as with VHF FM stereo, a certain amount of crosstalk is encountered, limiting stereo separation.

The stereo information is dbx-encoded to increase the signal-to-noise ratio (at low levels), to aid in noise reduction.

Typical S/N (signal to noise) is better than -50 dB.

The original specifications called for a brick wall elliptical filter in each of the audio channels prior to encoding. The cutoff frequency of this filter was 15 kHz to prevent any horizontal sync crosstalk from being encoded. Manufacturers of modulators, however, used lower cutoff frequencies as they saw fit. Typically, they chose 14 kHz although some used filters as low as 12.5 kHz. The elliptical filter was chosen for having the greatest bandwidth with the lowest phase distortion at the cutoff frequency. The filter used during EIA testing had a characteristic that was -60dB at 15.5kHz. As transformer audio coupling was common at that time, the lower frequency limit was set to 50 Hz although modulators without transformer inputs were flat down to at least 20Hz.

Typical separation was better than 35 dB. However, level matching was essential to achieve this specification. Left and Right audio levels needed to be matched within less than 0.1 dB to achieve the separation stated.

Maintaining the phase stability of the horizontal sync is essential to good audio in the decode process. During transmission, the phase of the horizontal sync could vary with picture and chroma levels. ICPM (Incidental Carrier Phase Modulation), a measure of transmitted phase stability, needs to be less than 4.5% for best audio sub channel decoding. This was more of a problem with UHF transmitters of the day. Multi-cavity klystron amplifiers of that time typically had an ICPM above 7%.

MTS licensing

Because of the use of dbx companding, every TV device that decoded MTS originally required the payment of royalties, first to dbx, Inc., then to THAT Corporation which was spun off from dbx in 1989 and acquired its MTS patents in 1994; however, those patents expired worldwide in 2004.[3] Though THAT now owns some patents related to digital implementations of MTS, a letter from THAT to the National Telecommunications and Information Administration in 2007 confirms that no license is required from THAT for all analog and some digital implementations of MTS.[4]

How MTS audio channels are used

The second audio program (SAP) also is part of the standard, providing another language, a video description service like DVS, or a completely separate service like a campus radio station or weatheradio. This sub-carrier is at 5x horizontal sync and is also dBx encoded.

A third PRO (professional) channel is provided for internal use by the station, and may handle audio or data. The PRO channel is normally used with electronic news gathering during news broadcasts to talk to the remote location (such as a reporter on-location), which can then talk back through the remote link to the TV station. Specialized receivers for the PRO channel are generally only sold to broadcast professionals. This sub-carrier is at 6.5x horizontal sync.

MTS signals are indicated to the television receiver by adding a 15.734 kHz pilot tone to the signal. The MTS pilot is locked or derived from the horizontal sync signal used to lock the video display. Variations in phase or frequency of the horizontal sync are therefore transferred to the audio. UHF transmitters in use in 1984 generally had significant phase errors introduced in this signal making the transmission of stereo audio on UHF stations of that time nearly impossible. Later refinements in UHF transmitters minimized these effects and allowed stereo transmission for those stations.

Most FM broadcast receivers are capable of receiving the audio portion of NTSC Channel 6 at 87.75 MHz, but only in monaural. Because the pilot tone frequency at 15.734 kHz is different from that of the ordinary FM band (19 kHz), such radios cannot decode MTS.

MTS and the DTV transition in the United States

As a component of the NTSC standard, MTS is no longer being used in U.S. full-power television broadcasting after the June 12, 2009 DTV transition in the United States. It remains in use in LPTV and in analogue cable television.[citation needed] All coupon-eligible converter boxes (CECBs) are required to output stereo sound via RCA connectors, but MTS is merely optional for the RF modulator that every CECB contains. NTIA has stated that MTS was made optional for cost reasons;[5] this may have been due to a belief that MTS still required royalty payments to THAT Corporation, which is no longer true except for some digital implementations.[4]

THAT has created consumer pages on the DTV transition and how it affects MTS.[6] The site describes the situation by stating that most consumers with CECBs will end up with monaural TV sound, since RF-only connections are common and MTS is optional (and rare) for CECBs.

See also


7. Alan Skierkiewicz,"Stereo one year later", Television Broadcast, November 1985, p76