What could cause temporary degradation of voice quality?

Q. What could have caused the degradation of the voice on IP site Connect, so during the reception of the message, part of it is incomprehensible for a few seconds, then it continues with the reception of the clear message. In the system we use SLR8000 repeater, DP and DM4600 series of devices.

A. In my experience, the most likely causes are: on-channel interference; intermodulation at the repeater site; loss of good signal due to desense or bad connections; or IP connectivity problems.

The radio and repeater model will not make a difference.

On-channel Interference

Depending on how strong it is, on-channel interference can either result in poor audio quality; loss of audio or unexpected call termination. On-channel interference can either occur by mistake or on purpose.

Some examples of accidental on-channel interference include:

• A licenced radio user, who has been allocated the same frequency, has moved outside their normal operational area and is trying to contact other users on their system.
• A licenced radio user, who is located close by and has been allocated the same frequency from the regulator by accident.
• A licenced radio user, whose radios were incorrectly programmed onto your frequency. Although extremely rare, it has happened before. For example, many CCC VHF radios are supplied with 145MHz as a default transmit and receive frequency. Many of these radios lend up with consumers who then use them as-is (unknowingly without a licence). 
• Unusual propagation conditions are allowing signals from a licenced user to travel long distances and reach your system . like this; this or this.

Deliberate interference would originate from someone who wants to cause problems, either for their own enjoyment or as a means to achieve something nefarious (a.k.a. jamming). This is rare but people are people ...

Intermodulation

Intermodulation typically occurs in radio systems where two or more signals pass through nonlinear components such as amplifiers; contacts between dissimilar metals or rusty contacts. When this happens, the hardware can generate additional signals (products) that are the sum and difference of the original frequencies.

These unwanted intermodulation products can be equal to the receive frequency of your radio system causing interference and degradation of the desired signal whenever the source signals are present.

For example a repeater with transmit frequency 145,6000MHz can produce an intermodulation product in 169,3000MHz whenever a nearby base operating on 121,9000MHz transmits. There are around 27 possible intermodulation products generated whenever these two signals are present and the conditions are right. If all of the equipment is configured and installed correctly, the level of these interfering signals will be low.

To mitigate the effects of intermodulation, one needs to include additional filtering between the radio equipment and antenna. This equipment isolates signals from each other, and suppresses the unwanted intermodulation products.

Also see here and here.

Desense

Desense is the degradation in sensitivity due to noise sources, which are either generated by the equipment itself (e.g. transmitter desenses the receiver) or by other equipment on the same RF site (e.g. transmitter on 145,6MHz makes repeater receiving on 148,325MHz go deaf).

A receiver has a certain minimum detectable signal which is determined by thermal noise - this is the noise the electronic components produce under normal conditions. When a signal is received, additional spurious signals are produced within the receiver because it is a non-linear device. 

Normally, these spurious signals are so low, they have no impact on the received signal quality.

An interfering signal can produce spurious signals. If the spurious signal level is stronger than minimum detectable signal, then the receiver becomes desensitized.

A method to test for desense is described in this post. Basically, the receiver sensitivity is tested using an isolated-tee bridge, first with a dummy load connected to one leg then replaced by the antenna. If no desense is present then the measured sensitivity with the dummy load and antenna will be the same or very close.

IP connectivity

IP connectivity would tend to only affect a single RF site served by the problematic connection. In order to pass voice reliably, a repeater in an IP Site Connect system needs around 16kbps with a peak latency of around 300ms and a jitter of around 60ms. Usually IP connectivity problems would be seen as missed calls or gaps in the received audio.

A 15 site, IP Site Connect network, with both timeslots wide, will need around 565kbps bandwidth. What is important to note here is that this bandwidth is mostly upstream.

Anything else?

If I was into gambling, I'd put my money on intermodulation or desense since, from experience, these have been the most common causes.