The audio sounds bad, why?

Although the overwhelming majority (>99,9%) of MOTOTRBO systems go in without any problems, about once or twice a year I get a complaint about this. In almost all cases, the end customer has just migrated from analogue. In almost all of these cases, many obvious things were overlooked and some effort is then needed to improve the audio fidelity and satisfy the customer.

This is based on my own experience with MOTOTRBO but most of what you read here applies to other makes of radios. Remember that all DMR radios will offer the same audio fidelity so this is not something specific to Motorola.

First, some facts:

• The AMBE+2 vocoder from DVSI was chosen by all DMR Association members and is used in all DMR radios.
• This vocoder offers the best performance at the compression ratio used in DMR, with some degradation of reproduction fidelity (see below).
• This vocoder consequently permits: two-timeslot operation on a single 12,5kHz channel; background noise reduction and enhancements like AES encryption; control and data.

The above video is a comparison of the audible differences between commonly used CODECS. As you can see, DMR uses a 50:1 compression ratio – which is half of what TETRA uses and one fifth of what GSM uses.

• DMR will, in most cases, outperform analogue in weak signal conditions. Generally, there will be a 13-84% improvement of coverage, depending on whether the call parties are stationary or moving.
• The scientific measurement of audio quality is PESQ (defined in ITU-T P.862 (02/01)). The scale is 1 to 5 – Poor to Excellent.
• An analogue two-way radio system with good signals will have a PESQ of 3,889.
• A MOTOTRBO radio system, operating in the same conditions as the analogue system, will have a PESQ of 3,071.

Therefore we can say that AMBE+2/MOTOTRBO is responsible for the loss of 0,818 PESQ units when compared to analogue. We can also say that the vocoder performance is limited by the channel bandwidth and consequently bit rate. Remember that all DMR radios will offer the same audio fidelity.

Here is a Pareto chart showing the root causes for poor audio quality in MOTOTRBO and their respective incidences. The values are based on the actual number of support requests I've seen over the recent years relating to audio.

As you can see, the top three causes have nothing to do with hardware or configuration:

1. Setting the wrong expectations means that the customer somehow has the expectation that digital will sound better than analogue. When many end customers hear the word “digital” they automatically think of things like MP3 players; CD or DVD or Internet Radio. While all of these are indeed digital, the audio fidelity is quite different (due to bandwidth and bitrate). In many cases, the radio should really be sold with an appropriate accessory.
2. MOTOTRBO will outperform analogue in high noise conditions – this is because the AMBE+2 vocoder is specifically designed to filter out noise and only pass voice. Have a look at this video - it shows that MOTOTRBO performs significantly better under high noise conditions. In order to achieve adequate noise cancellation, the radio needs >6dB voice to noise ratio – below this, the radio starts to struggle with capturing sufficient voice and the resultant audio quality will suffer.
3. Similarly, shouting or speaking too softly into the radio will result in poor audio performance. In high noise environments, where hearing protection is insufficient, radio users will experience the Lombard reflex and start to shout at others and into the radio. Users need to be educated (and re-educated) on how to correctly use the radio. This video has some really good tips even though it was intended for use by Fire/Rescue personnel using P25 radios.

Generally users need to speak in a normal tone, at a normal pace and hold the radio/RSM 5-10cm from their mouth. In high noise environments, the radio can be held closer (i.e. 2-5cm). If the radios are being used in a high noise environment, where hearing protection is compulsory for health and safety reasons, it makes sense to consider using one of the many audio accessories available for the radios.

In some cases, where a speaker-microphone cannot be used, the Enhanced Noise Cancelling* feature can be enabled in the radio to provide additional noise cancelling. The Active Noise Cancelling remote speaker-microphone provides the highest level of background noise cancelling and is suitable for users who work in extremely noisy environments where regular Noise Cancelling Speaker-Microphones would not ideal.

In R2.4.0 (R1.12.0 for Gen1 radios), an improvement was made to the AGC algorithm. One of the checks would be to ensure the radio is on the latest firmware, or at least something newer than R2.4.0. Radios from the factory will almost always be supplied with the latest firmware. In some cases, an upgrade would be needed to enable useful features like Trill Enhancement.

Good or bad, analogue was very forgiving in terms of audio fidelity. With the right usage habits; correct system configuration and the proper accessories, a MOTOTRBO system can perform as good as - or even better - than an analogue system.