# AM & FM Radio Questions **URL:** \ **Category:** Factual Questions\ **Created:** [September 25, 2005, 7:52pm UTC](https://boards.straightdope.com/t/am-fm-radio-questions/323366 "2005-09-25T19:52:51Z")\ **Posts on this page:** 3\ **Page:** 2
**Author:** ![Airman\_Doors\_USAF](https://avatars.discourse-cdn.com/v4/letter/a/e36b37/32.png) [@Airman\_Doors\_USAF](https://boards.straightdope.com/u/Airman_Doors_USAF)\ **Post date:** [September 26, 2005, 7:15pm UTC](https://boards.straightdope.com/t/am-fm-radio-questions/323366/21 "2005-09-26T19:15:38Z")
> [@engineer\_comp\_geek](#): > > Television uses AM for the video channel and FM for the voice channel. It’s not using both modulation techniques on the same signal. Yes it is. It’s a composite signal. You can break out the video and audio carriers separately, but the signal is created within the same modulator. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [September 27, 2005, 2:04am UTC](https://boards.straightdope.com/t/am-fm-radio-questions/323366/22 "2005-09-27T02:04:39Z")
> [@Airman Doors](#): > > …the signal is created within the same modulator. No, it is not. The sound in a US NTSC television signal is modulated separately from the video on a separate carrier spaced 4.5 MHz up from the center frequency of the video carrier. ---
**Author:** ![Antonius\_Block](https://avatars.discourse-cdn.com/v4/letter/a/51bf81/32.png) [@Antonius\_Block](https://boards.straightdope.com/u/Antonius_Block)\ **Post date:** [September 27, 2005, 10:12am UTC](https://boards.straightdope.com/t/am-fm-radio-questions/323366/23 "2005-09-27T10:12:42Z")
> [@Derleth](#): > > Is there any scheme that modulates both frequency and amplitude? Would that even be useful? > … > I meant using AM+FM to add extra information to a pure audio broadcast. An audio-only application that uses both frequency- and amplitude-modulation (but not on the same carrier) is FM stereo radio. By far the most common implementation is the [Zenith/GE pilot-tone multiplex system](http://en.wikipedia.org/wiki/FM_radio), which involves a sum signal (L+R channels, to provide compatibility with mono radios) up to ~15kHz, and a difference signal (L-R) **amplitude-modulated** onto a 38kHz carrier. The 38kHz carrier is suppressed before the transmitter to reduce the RF power required to transmit a given audio level. A lower-level 19kHz pilot-tone, phase-locked to the original 38kHz carrier, is added to the modulating signal, and recovered via a phase-locked loop in the customer’s stereo receiver. The RF carrier frequency is then **frequency-modulated** by the overall signal containing:[ul] [li]0 to 15kHz: (L+R) mono audio.[/li][li]19kHz: pilot tone.[/li][li]23kHz-38kHz: lower sideband of (L-R) audio.[/li][li]38kHz-53kHz: upper sideband of (L-R) audio.[/ul][/li]The bandwidth alloted to an FM stereo broadcast is greater than for AM mono for at least three reasons: [ol] [li]The audio bandwidth is 15kHz, compared to 10kHz (or less).[/li][li]The addition of the (L-R) channel requires additional bandwidth for the modulating signal, up to 53kHz total in the standard implementation.[/li][li]Frequency modulation is _inherently_ higher-bandwidth than amplitude modulation. The frequency spectrum of a carrier _amplitude-modulated_ by a sinewave contains just the central carrier (f[sub]carrier[/sub])and two components at (f[sub]carrier[/sub]+f[sub]modulator[/sub]) and (f[sub]carrier[/sub]-f[sub]modulator[/sub]), whereas the spectrum of the same carrier _frequency-modulated_ by the same sinewave contains the central carrier (f[sub]carrier[/sub]), plus paired components at (f[sub]carrier[/sub]+/-f[sub]modulator[/sub]) as with AM, and also at (f[sub]carrier[/sub]+/-2f[sub]modulator[/sub]) and (f[sub]carrier[/sub]+/-3f[sub]modulator[/sub]) and so on to infinity (although by far most of the energy is in the first two harmonics, so the RF is bandwidth-limited before transmission and FM stereo radio stations can be spaced 200kHz apart on the dial without much interference).[/li][/ol] As would be expected, it takes a higher bandwidth to transmit and receive an FM stereo signal than an FM mono signal. In fringe conditions stereo FM may be noisy, and bandwidth-limiting the detector to pass only the (L+R) signal may provide a better listening experience (“_the wider you open the window, the more dirt flies in_”). [Previous page](https://boards.straightdope.com/t/am-fm-radio-questions/323366.md?page=1)