L+R is the common technical description for the audio mixing of left and right stereo channels to produce a monaural channel.
Most AM broadcasting is straight monaural, but the source audio frequently comes from a stereo audio source. For this purpose, the AMT3000 and 5000 transmitters from SSTran include two audio inputs, providing an on-board L+R mixer.
In FM stereo broadcasting one must keep in mind that listeners with 1-channel mono FM receivers will be hearing an L+R mix.
For crystal clear monaural audio the L and R channels must agree in phase relationship. If one channel gets flipped in polarity the result will be cancelation in the L+R mix, sounding like a cheap telephone call at the listener's end, yet stereo listeners will hear normal sound.
This same rule applies also in the recording process, to safeguard the integrity of the recorded sound for the time when it gets heard in a mono mix.
In the very early days of stereo LP records there were frequent releases that sounded great in stereo, but canceled in mono because technicians hadn't corrected for proper polarity. I have such a recording from Decca featuring the Bert Kaempfert Orchestra.
Veteran radio engineers have often experienced out-of-phase broadcasts on both radio and television, and yet newcomers to the field are still learning this lesson the hard way.
Thanks for the reminder for some and new information for others. This one is easy to overlook and can cause audio trouble.
Neil
You can get the same problem with these recording on fm if the fm receiver is mono or out of range for a stereo reception.
These these sort of recording's I mix down to mono making sure the phasing is right.
Johny reminds me of steps I've taken in the past to deal with phase control.
While I had a professional recording/broadcast studio everything was inter-connected with balanced audio wiring, which makes it easy to flip-phase on one channel only to correct for error.
Each turntable and tape deck was equipped with a polarity-flipping switch on the right channel.
My loudspeaker monitor system was able to present stereo or mono at the throw of a switch. If the mono showed cancelation coming from a record I'd flip the polarity of the right channel and it was fixed, but I had to remember to put the switch back in the normal position.
Fortunately very few out-of-phase record albums are around, but like Johny said, out of phase tapes can be done wrong by whomever produced them.
Also, in a nearby thread on fine-tuning AM antennas, Neil mentioned 2-channel scopes, and I had a 2-channel oscilloscope for fine-tuning the phase azimuth of heads on tape decks. Sometimes other people's tapes were produced with poor azimuth (tape-head-alignment).
By the way, out-of-phase audio can be used to cancel the vocalist and come up with an instrumental track of a song.
Tuning around in the off-azimuth direction can yield the flanging effect heard on some special-effects hits.
Rememeber the early mono to stereo converted Beatles albums? Music in one side, vocal in the other.