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<title>Channel memory</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Channel memory</span></span>

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<p>An <b>automatic channel memory system</b> (<b>ACMS</b>) is a system in which a digitally controlled <a href="Radio_tuner" class="mw-redirect" title="Radio tuner">radio tuner</a> such as a <a href="TV_set" class="mw-redirect" title="TV set">TV set</a> or <a href="VCR" class="mw-redirect" title="VCR">VCR</a> could search and memorize <a href="TV_channel" class="mw-redirect" title="TV channel">TV channels</a> automatically. While more common in <a href="Television" title="Television">television</a>, it can also be used to store <a href="https://en.wiktionary.org/wiki/preset" class="extiw external" title="wikt:preset">presets</a> for <a href="Radio_station" class="mw-redirect" title="Radio station">radio stations</a>. This is often called a <b>channel scan</b>,<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> though that may also refer to a "<a href="Preview_(computing)" title="Preview (computing)">preview</a>" mode which plays each station it finds for a few seconds and then moves on to the next, without affecting <a href="Computer_memory" title="Computer memory">memory</a>.

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<div class="mw-heading mw-heading2"><h2 id="Channel_scanning">Channel scanning</h2></div>

<p>A typical TV device allows an automatic channel scan to be performed from a <a href="Menu_(computing)" title="Menu (computing)">menu</a> accessed by a button on the TV set, or sometimes only on the <a href="Remote_control" title="Remote control">remote control</a>. This applied first to <a href="Analog_TV" class="mw-redirect" title="Analog TV">analog TV</a> sets — sometimes those with digital <a href="LED_display" title="LED display">LED displays</a>, or later always those with <a href="On-screen_display" title="On-screen display">on-screen displays</a>. These simply searched for the video <a href="Carrier_wave" title="Carrier wave">carrier</a> signal on every channel. (Before the advent of ACMS, many sets would search for the next channel every time it was changed.)

</p><p>It now also applies to <a href="Digital_TV" class="mw-redirect" title="Digital TV">digital TV</a>, which must not only find the signal itself, but also decode its <a href="Metadata" title="Metadata">metadata</a> enough to <a href="Virtual_channel" title="Virtual channel">remap channel numbers</a> to their proper locations. In the case of the American <a href="ATSC" class="mw-redirect" title="ATSC">ATSC</a> system, the <a href="ATSC_tuner" title="ATSC tuner">ATSC tuner</a> uses <a href="PSIP" class="mw-redirect" title="PSIP">PSIP</a> metadata to do this. The internal <b>channel map</b> for digital <a href="TV_station" class="mw-redirect" title="TV station">TV stations</a> is different from the presets or "<a href="Favorites" class="mw-redirect" title="Favorites">favorites</a>" that the user has programmed. Just as with analog TV (which worked <i>only</i> by turning a preset on or off for each station/channel), users of <a href="Digital_television_adapter" title="Digital television adapter">digital television adapters</a> and other similar tuners can choose to ignore channels that are still in the channel map.

</p><p>Analog station presets and digital channel maps are normally deleted when a new scan is started. On some tuners, digital channel maps can be added-to with an "easy-add" channel scan, which is useful for finding new stations without losing old ones that may be <a href="Radio_propagation" title="Radio propagation">weak</a> or currently off-air, or not aimed-at with an <a href="Antenna_rotator" title="Antenna rotator">antenna rotator</a> or other set-top <a href="TV_antenna" class="mw-redirect" title="TV antenna">TV antenna</a> adjustment. If a station adds a <a href="Digital_subchannel" title="Digital subchannel">digital subchannel</a>, most digital TV tuners will find it automatically as soon as the user turns to another channel that is carried by that station, adding it to the channel map.

</p><p>Many will also automatically add a new digital station's subchannels by tuning manually to the station's physical channel, though if this conflicts with the virtual channel number of another station, a complete re-scan may be the only solution. (Choosing an unused subchannel number [i.e. 30.99] on that major channel number [i.e. 30] may avoid the remap on existing subchannels [i.e. 30.1] and force the tuner to listen on that physical channel.) This has often happened in the U.S., where stations (especially <a href="LPTV" class="mw-redirect" title="LPTV">LPTV</a>) find it easiest to place their digital operations on a vacated analog channel.

</p><p>The same problem also occurs when the same station moves its digital transmission back to its old analog channel. There is no way to delete a station from the internal channel map, and a re-scan may be needed; or if the new physical channel is found, it may leave the old mapping in place, causing duplicate channels that cannot be accessed through direct entry of the numbers without also pressing the channel-up/down buttons.

</p><p>Many modern TV sets do an ACMS scan automatically as part of the setup process the user is guided through when the device is initially plugged-in. While early sets often lost their memory in a <a href="Power_outage" title="Power outage">power outage</a> or by otherwise being disconnected from <a href="Mains_electricity" title="Mains electricity">mains electricity</a>, all of them now have <a href="Non-volatile_RAM" class="mw-redirect" title="Non-volatile RAM">non-volatile RAM</a> or a backup of some sort.

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<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFHeinzman2019" class="citation web cs1">Heinzman, Andrew (19 June 2019). <a rel="nofollow" class="external text" href="https://www.howtogeek.com/424853/how-to-scan-or-rescan-for-channels-on-your-tv/">"How to Scan (or Rescan) For Channels on Your TV"</a>. <i>howtogeek.com</i><span class="reference-accessdate">. Retrieved <span class="nowrap">25 January</span> 2025</span>.</cite></span>

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This article is issued from <a class="external text" title="Last edited on 2025-01-25" href="https://en.wikipedia.org/wiki/?title=Channel_memory&oldid=1271755478">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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