The Utah VHF Society

Channel Spacing for Yaesu C4FM<>C4FM and C4FM<>Analog FM signals

Purpose of this page:

As new technologies come into use on the amateur bands, there is an increasing challenge to be able to evaluate and support these technologies. In the past, conventional test equipment has been used to maintain and diagnose such systems, but with these new technologies there is a challenge to be able to provide a means of being able to support such systems in a meaningful way.

One of the recent additions to the list of technologies for conveying low-rate voice and data is Yaesu's C4FM as found on their DR-1 repeater, FT-1DR Handie-Talkie and the FTM-400DR mobile radio.

Important Notes:

What is C4FM and how does it compare to D-Star?

As transmitted on-air, a C4FM signal - like Icom's D-Star signal - is simply FM:  More specifically, it uses Frequency-Shift Keying (FSK) to convey data.  By properly shaping the modulating waveform and appropriately choosing the amount of deviation, the transmitted spectrum can be adjusted to minimize the occupied bandwidth while still maintaining reasonable power efficiency in terms of being able to transmit data.

The D-Star system implemented on the amateur bands by ICOM uses a 2-level modulation scheme:  In terms of a raw signal, a low frequency might represent a "0" while a high frequency may represent a "1".  What this means is that for each symbol transmitted, we get ONLY a "0" or a "1":  In other words, we get 1 bit per "baud" and for 4800 baud modulation used with D-Star, our raw data rate is 4800 bits per second.

C4FM's modulation is slightly more complex.  Instead of just a "0" or a "1" being transmitted by a low and high frequency, respectively, there are actually four possible states:
During any single baud period, the signal could be at any one of the above four states.  Like D-Star, the baud rate is 4800 symbols per second, but since each symbol could represent any one four bit combinations - 00, 01, 10 or 11 - we are actually conveying two bits per baud.  What this means is that at just 4800 baud, our raw data rate is actually twice that of D-Star, or 9600 bits per second.

C4FM used by the Yaesu gear is comparable to the C4FM used with Phase 1 P25 ("Project 25") radios - See the Project 25 page on Wikipedia for more information.

We can't get something for nothing!

While we can send 2 bits per symbol using C4FM, that doesn't mean that there isn't a compromise.  When we send just one bit per symbol - as in D-Star's MSK - we have only two possibilities:  A "0" or a "1".  With C4FM we have four possibilities which means that in the event of a noisy signal, our receiver is going to have a more difficult time determining if the symbol received was a "00", "01", "10", or a "11"  and this is due to the decreased "distance" between the four possible states of C4FM as opposed to the two states of MSK used in D-Star.  According to Shannon's Law, the more data we pack into a smaller period of time, the more bandwidth and/or energy (power) we will need to convey it if we wish to maintain the same error rate, so something must be done with the C4FM signal to make it work well with weak, noisy signals.

In order to enhance the ability to decode C4FM signals with fewer errors, here are but two of possibilities:

Overlay of C4FM, FM and CW signals

Of those listed, above, I am only certain of the aspects of the first - that is, increasing the modulation index as I have yet to look at a technical manual for the Yaesu C4FM gear to see what is used for demodulation.

Occupied bandwidth of the C4FM signal:

The question to be answered here is "How much bandwidth does a C4FM signal need?"  The answer, as implied by the discussion above, is that more bandwidth is required than for D-Star, but how much?

Figure 1 shows a composite of several signals, from the same transmitter, to help one divine an answer.
What is immediately apparent is the fact that the occupied bandwidth of the analog FM and C4FM signals are very similar.

If one compares these with plots of a D-Star signal (see Figure 1 on the page about D-Star Channel Spacing (link) on this web site) it is apparent that the C4FM signal is significantly wider - approximately 12.5 kHz at the -26 dB points as compared to approximately 10 kHz at the -26dB points for D-Star, both measurements being relative to the peak of the modulated carrier.

Comparing the bandwidths of the signals with respect to an unmodulated carrier would require stating the resolution bandwidth of the measurements owing to the power density of the bandwidth-limited noise aspects of these digitally-modulated carriers.  As long as the resolution bandwidth is a fairly small fraction of the total occupied bandwidth of the digital signal being measured we can provide fairly consistent and repeatable measurements.
This analysis and comparison allows us to come to several conclusions and speculations as to the rationale behind these differences:
"Digital signals make better neighbors"

When comparing a digital signal to an analog one, there is more than just occupied bandwidth to be considered, but also the way that the bandwidth is actually used.  When either a C4FM or D-Star signal is active, it is, for practical purposes, bandwidth-limited noise and owing to the nature of the digital signal, the way the energy is spread over the occupied bandwidth is fairly constant over time.

An analog signal, on the other hand, is quite variable.  As can be seen from Figure 1, an unmodulated carrier (e.g. silence) consists of a "spike" with most of the power sitting on the nominal transmit frequency.  With the wildly variable nature of the human voice and the sounds that it can make, the occupied bandwidth of an FM signal can vary from just that single spike to broad bursts of noise over quite a wide bandwidth from the sounds of consonants and this can be difficult to represent on a static spectrum analyzer plot!

What this means is that with normal (analog FM) voice there will be periods of very narrow bandwidth corresponding to silence and quiet parts of speech as well as very wide bandwidth bursts that last only milliseconds - but this has an implication for neighboring frequencies:  While an adjacent analog signal will occasionally have bursts of "splatter" that will very briefly impact a close, adjacent-frequency signal, a digital signal - with its fairly consistent modulation - will deviate very little from its nominal bandwidth and will thus be a better "neighbor" than an analog one!

Receive bandwidth:

While the bandwidth occupied by the transmitted signal is important, on a radio system we must take into account the bandwidths of the receivers typically
being used.  If we had a hypothetical digital signal that was just 5 kHz wide - but our receiver was 20 kHz wide - while we could place two of those 5 kHz signals 10 kHz apart with room to spare, our 20 kHz wide receiver would not be able to distinguish the two!

Through empirical measurements of various D-Star gear, we determined that the typical receiver in D-Star mode was approximately 11 kHz wide at the -30dB points.  Considering that the vast majority of the transmitted energy of a D-Star signal is contained within less than 10 kHz of bandwidth, this was a reasonable compromise between practical and affordable IF filtering and minimizing degradation of the received signal by the filters themselves.  In our analysis, we determined that a "safe" spacing for D-Star signals - considering the occupied bandwidth of the transmitted signals, the bandwidth of the receivers that would be used - as well as expected frequency tolerances - was 12.5 kHz.

Clearly, the C4FM signals are wider than the D-Star signals - quite comparable to analog FM signals - so this means that we require a wider channel spacing.  In measurements with actual C4FM gear, it was determined that the IF filtering bandwidths appeared to be the same as those for normal "wide-band" (+/- 5 kHz) FM signals.  (Specific measurements of actual C4FM gear receive bandwidth will follow.)

(Preliminary) Conclusions:

At the present time we have yet to run actual co-channel interference tolerance tests to determine how adjacent C4FM signals interact with each other or, more importantly, the degradation of an analog signal by an adjacent C4FM signal (and vice-versa) - This testing will follow.

At the present time we can conclude the following with good confidence:

This page is a work in progress will be updated.


This matter is open for discussion:  If you have concerns or opinions one way or another, please make them known to the frequency coordinator at the email address below.

Questions, updates, or comments pertaining to this web page may be directed to the frequency coordinator.

Return to the  Utah VHF Society home page.

Updated 20140227

Page count since 20140303: