I prefer how digital voice sounds over a P25 system vs analog (MOTOTRBO has come a long way but isn't quite there).
It likely will never sound as smooth as P25. There's some important differences to note; some of which you may already be familiar with, but I'll share here so others can understand.
P25 Phase I (conventional and trunking) uses 9600bps of data on a Frequency Dvision Multiple Access (FDMA) channel. Of that 9600bps, 4400bps is used by the Improved Multi-Band Excitation (IMBE) vocoder, while 2800bps is used for Forward Error Correction (FEC) and another 2400bps is used for embedded data, to send information such as the Radio ID (RID), GPS information, Soft ID, encryption algorithm/KID ID, manufacturer ID, etc. (4400 + 2800 + 2400 = 9600).
DMR uses 4800bps of data on a two-slot Time Division Multiple Access (TDMA) channel. Of the 4800bps, 2450bps is used by the Advanced Multi-Band Excitation +2 (AMBE+2) vocoder, with 1150bps used for FEC and 1200bps is used for embedded data. Since there's two time-slots, the total combined bit-rate for both is 9600bps.
The key differences between P25 Phase I and DMR are the vocoders and the bit-rates. AMBE+2 is newer and is extremely efficient at low bit-rates and handles background noise (unwanted crap audio) much better than IMBE. It wins hands down for getting rid of noise associated with road/vehicle/industrial/machinery/factory/engines, etc. That said, the P25 IMBE vocoder runs at nearly double the bit-rate and has better fidelity because of this. But it also takes up an entire RF channel -- not efficient by today's standards.
I did extensive work last summer to improve the audio quality of the MOTOTRBO platform. It does sound much better now. First-gen products still need the changes ported to a new template/firmware release. That's not my doing; I do not work for Motorola. I only consulted and sent the fixes to them. They were implemented in R02.50.xx in second-gen products. Motorola also included some of their own enhancements. Overall, 2.50 is a major improvement. Digital AGC must be enabled to take full advantage of the enhancements.
I wish there wasn't such a delay when you talk over a digital system. Makes it very hard for me to concentrate if I'm hearing my voice delayed a second in the background because someone didn't turn their radio down.
I know you know this, but for mouth-breathers reading this, the solution is for people working in close proximity to use proper audio accessories. i.e. in-ear solutions. No howling, no digital echos. Ideal solution, but rarely happens in the real world. Too many people with deviated septums.
I try to educate my co-workers but it goes in one ear and out the other and they blame the radios and system.
They're just too stupid to use the radios properly, that's all. Take pride in knowing you're trying to improve the issues and make things better.
I've also found that there are so many more deadspots with my employers MOTOTRBO system vs analog and even the cities P25 system. I get frustrated when I need to reach dispatch and I get the tone that I can't hit the repeater.
Have you ruled out programming errors? Setting the Admit Criteria to "Color Code Free" if it's currently set for "Channel Free" could resolve some of those issues.
You may also consider enabling "Enhanced Channel Access" if you're using a stand-alone or IPSC digital channel. That will prevent users from talking over each other (or keying up at the same time), which will cause missed transmissions and packet loss.
In my testing, I've found DMR useful until signals reach about -122 to -124 dBm. That's very acceptable. You may wish to investigate the noise floor of your RX channel at the repeater site. Interference/site-noise affects analog signals differently than digital (packet loss, instead of popping/crackling). Consider a preselector on your RX channel. It will knock noise down about 4-6 dB. There are several Motorola preselectors on eBay these days, for very decent prices.
If your idle noise floor (analog or digital) is higher than the weakest workable DMR signal, you're losing range. (ex.: noise floor of -115 dBm is masking a workable -120 dBm DMR signal). A good preselector will have an insertion loss of less than 1.2 dB. You lose that, but gain the 4-6 dB back on your quieter noise floor.
I shouldn't have to rely on my cell phone to talk to my dispatchers especially in an emergency. I could rant forever on this topic but my goal here is to equal out digital vs analog Rx levels on my 5550.
The radio system will only work as well as it's maintained. Infrastructure maintenance/configuration is equally important as subscriber programming/configuration/training. Document your system/user complaints and investigate the causes. Improvements can be made; some easier to implement than others.
Subscriber programming is the most overlooked component in system issues/complaints.
