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I am for the first year in electrical engineering so sorry for this question but i couldn't test it by my own.

One of my lecturers said that the remote control of a TV saves the channel we choose, and that when we press the "last channel" button, the remote sends the channel and not the "return" signal. Is this true?


Thank you all for your answers, they were all good ways to test it. I will inform my instructor about them!

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    \$\begingroup\$ I guess that depend of the remote and the tv. If the remote keep in memory previous action, then it return the last send button. If the tv saves the last action, then the remote send "previous" \$\endgroup\$ – M.Ferru Jun 2 '17 at 20:28
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    \$\begingroup\$ My bet is that the TV remembers the last channel and not the remote because remotes are often extremely low-cost and simple devices while a TV needs to be more complex anyway so implementing a "previous channel" feature costs virtually nothing in a TV. Also, the remote does not "know" if the TV has received the last "set channel" command but the TV does. So the TV always know which was the previous channel while the remote can only "assume". \$\endgroup\$ – Bimpelrekkie Jun 2 '17 at 20:58
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    \$\begingroup\$ Your lecturer is a moron. Probably why they are a lecturer and not a professor. \$\endgroup\$ – Passerby Jun 2 '17 at 22:09
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No. Absolutely not. The "Last/Return/Previous Channel" button is a unique function code that the TV interprets!

For the most common IR control protocols, Philips' RC5, Sony's SIRC or NEC, the remote is normally an application-specific integrated circuit (ASIC) with hardcoded TV codes for fixed remotes or a microcontroller with a reprogrammable eeprom for learning remotes (programmed via special codes or through a programming interface like JP1 or even IR learning).

The remotes have a single code programmed into each button, and that does not change without intentional reprogramming. These codes are arbitrary for the remote and depend on the TV to interpret. You can see an example for Insignia TVs here: http://www.getzweb.net/jp1/data_returns/DeviceEFCs.php?type=TV&devid=1204&webpid=++396

Each of these codes are single function, unless you get into more advance macros which are a group of codes assigned to the same button that get sent in order.

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Unlike some of the dubious test methods listed above, you can actually decode the actual code sent by your remote. All you need is a common IR receiver module and a microcontroller (MSP430, Arduino) or computer like the Raspberry PI.

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Once you do, you will find that the "Previous" button will always send the same exact code, regardless of what channel you have switched to and when. The remote does not actively track what channel or ANYTHING you do.

A different way would be to use a IR Learning Remote like the Harmony remotes. If the previous button was an active copy of the last channels selected, then you could not clone the feature.

Further note, the On/Off button is typically a single code that the TV interprets as toggling the current state, but discrete On and discrete Off codes often exist for any given tv, as well as dedicated "Input Source X" button instead of "Next Input" type codes.

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It might be true, but it could just as well work the other way around too. Probably real systems do this different ways.

I know that my TV has non-volatile memory in it, since it powers up to the same channel it was shut off with, without the remote being active.

Here is a simple way to test this:

  1. Use the remote to set the TV to channel A.

  2. Then use the remote to go to channel B.

  3. Bring the remote to a different closed room so that it's IR can't get to the TV.

  4. Select channel C on the remote.

  5. Select channel D on the remote.

  6. Bring the remote back to near the TV.

  7. Selet last channel.

  8. If the TV goes to channel C, then the last channel is stored in the remote. If the TV goes to channel A, then the memory is in the TV.

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One of my lecturers said that the remote control of a TV saves the channel we choose, and that when we press the "last channel" button, the remote sends the channel and not the "return" signal. Is this true?

Almost certainly not.

Outside of complicated "smart" remotes, most remote controls are very simple devices. They have one signal associated with each button -- the TV is responsible for deciding what to do with that signal. They do not keep track of the current channel in any way.

For a remote control to function the way your lecturer described, it would probably need to be in two-way communication with the TV, so that it can be aware of things like what channel the TV was in when powered on, when channels are changed using buttons on the TV, what channels are available in your area… it'd be a much more complex design, with absolutely no benefit to the end user.

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From my understanding I don't believe that a TV remote saves any information as it has no memory. When a button is pressed a configured binary value is generated which is then combined onto a carrier wave which will then be sent to the TV unit through a IR LED.

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Simple way to test this is to select a channel, change it, then hit "Last Channel" to confirm that it works. Then remove the batteries from the remote for about 1 minute (just in case), put them back in and hit "Last Channel" again. If the remote stores that info, it will do nothing, if the info is stored in the TV and the remote simply sends a command to the TV to revert, it will go to the last channel.

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    \$\begingroup\$ Assuming its RAM and not NVRAM or EEPROM or other type of memory being used... This is a bad explanation/test. \$\endgroup\$ – Passerby Jun 2 '17 at 22:11
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An additional practical reason why remembering the last channel is an operation that would be carried out by the TV is there are many situations where people may use multiple remotes and having a "Last Channel" per remote would be pretty annoying to the end user.

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