channel 47 (or any other channel) would be rejected.

We now also see why wide band receivers, which has an IF range of 435 to 475 KHz (40 KHz), can not operate at the same time with a transmitter one channel above or below them.

With this basic understanding of a single conversion receiver, let us expand this to a dual conversion unit. A dual conversion receiver does just as its name implies. It converts the incoming frequency twice. As with the single conversion, the IF is still a narrow tuned 455 KHz section. But instead of converting the incoming frequency directly to 455 KHz, it is converted twice, first to 10.7 MHz and then to 455 KHz. The receiver xtal in a dual conversion receiver is 10.7 MHz above the transmitter frequency and a second oscillator (which is not plugable) converts the 10.7 MHz to 455 KHz .

Dual conversion offers additional interference rejection and eliminates the 2IM interference caused by two other transmitters operating 23 channels apart which generates a 460 KHz beat frequency. The main reason a dual conversion receiver is not affected by 2IM is that all of the sum and difference frequencies that can be created in the 72 MHz band fall either below or above the 10.7 MHz first IF.
   

PPM vs. PCM: PPM (Pulse Positional Modulation) and PCM (Pulse Coded Modulation) are confusing term. The word modulation should be replaced with the word encoding. Both PPM and PCM encoding is modulated onto a carrier frequency and could use either AM or FM. PCM  is normally provided only on FM based systems.

In PPM encoding we saw that marker bits, which  are spaced the width of the servo pulse, is what is modulated onto the radio frequency. This means that the pulse sent by the RF module is of a time duration only. For example, if the signal generated by the elevator stick in the transmitter is 1.7 ms long then the PPM marker pulses will be 1.7 ms apart. The receiver then receives this 1.7 ms spaced signal, processes it and then sends it to the servos.

This encoding scheme is simple and effective, and has become the accepted method for RC for the last 20 or so years.  It has one serious drawback, and that is, if for any reason, interference alters the pulse train in any way, the Receivers decoder will become confused, and the servos would be sent wrong position data. This would produce erratic, random operation, commonly known as "glitching". 

With the advances in microelectronics, it became possible to design cost-effective PCM systems.  In a PCM system, a microprocessor (computer) replaces the encoder found in the transmitter of a basic PPM system. In the PCM system, control stick positions do not control pulse generators but are inputs into A/D (analog to digital) converters which convert the stick position into binary numbers, The microprocessor then assembles all the numbers from all the channels  into a data string, calculates a checksum, and outputs a data frame to the RF module for  transmission to the receiver.

Aside from sending a digital representation of the pulse width, the main difference between the PPM and PCM radios is the addition of the checksum. The checksum is nothing more than a "total" count of the binary codes generated for each channel within a specific frame. The receiver also has a checksum counter, and as the frame is received, the receiver's checksum counter "totals" the binary codes. Once a frame has been received, the receivers checksum is compared with the checksum sent by the transmitter. If the two totals are equal the receiver passes the processed channel data on to their respective servo. If the two totals from the checksum counters are different, the receiver discards that frame of data and the servo's hold their current position.

This means that the Receiver has a means to distinguish good and bad data frames, i.e., frames that have interference-caused glitches in them.  The Receiver will then reject bad frames, and maintain the servos at the last known good position.  This is known as "Hold". 

Some PCM radios have an added feature, called "Failsafe".  If the Receiver has not received at least one good frame in more than 1 sec. (this delay is