Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

Hi Adrian Help me understand why the need to use 4FSK compared to say FSK OR BPSK ? For voice at 700bps there are few bandwidth issues for amateurs... The multi level modulation (4FSK) will be inferior to others (without an time delay equaliser) when the long delay multipath is severe The

Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

There are some improvements that could be made (while using legacy FM demodulators). A common mistake is to tightly band limit the input the the modem. The problem with this is that the FM clicks- spikes spread out and cover perhaps several bits of information . The clicks have to be removed

Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

Hi Glen, We don't need to use the same chain for digital demodulation, since we are not software constrained. Using Gnuradio, we can simply reuse a flowgraph and connect/disconnect blocks on a click of a button to switch from one mode to another. What I don't want to do, is write a custom

Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

Ham modes/implementations that are not bandwidth critical should always do better than bandwidth constrained modes like DMR. consider the low deviation that DMR operates with... a symbol rate of 4800 sps and a maximum deviation of 1944 Hz, This is a comparatively low modulation index. This low

Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

On 9/12/17, David Rowe wrote: > > Have you performed any BER tests to conform the BER versus Eb/No > performance of the DMR waveform over ideal FSK? > > Cheers, > > David > Hi David, No I haven't done any BER tests yet. The code for BER is partly in place, but the 4FSK

Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

depends on the applied data/modulator filter prior to modulation. the trellis coding doesnt actually remove the isi it's all about pre filtering and post equalisation, directed rather than static. Though the DMR modem is super simple. Be aware, some modulation types are designed to have

Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

The ISI interpretation really only makes sense if you are using an analog FM demodulator for both waveforms. This is not the best way to demodulate FSK - it's a rough method - a kludge - that has somehow found it's way into a standard! BTW it's not really "David's modem" - Brady and I just

Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

Hi Adrian, Nice to see some one else working in this area. I imagine another reason is that the people setting the DMR standard were constrained by having a black box, closed source speech codec which constrained the bit rate they had to deal with. Have you performed any BER tests to

Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

Wouldn't the ISI largely (theoretically, completely) be ameliorated by trellis decoding? And I'm not talking about FEC, but rather incorporating the ISI into the expected Cartesian coordinates for each symbol. I did some experimentation with this technique on BPSK31, and I was able to get a

[Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

Hi, In this post: http://www.rowetel.com/?p=4650 David was asking the question why the 4FSK modem used by the DMR standard seemed to be performing so poorly, especially as compared to his ideal 4FSK modem. I was curious myself, so I implemented both and analysed them using my modified version of