MC70

Because the radio did not need to generate audio tones,  only MPT1327 signalling, the number and resolution of the DAC was reduced from 2 separate DACs with 4 resp. 3 bit resolution (in the non-trunked versions "EZA9" and "EVA9") to one single 2-Bit-DAC. This saves CPU pins, but allows only for about 12 dB in signal-to-noise-ratio (i.e. quantisation noise is only about 12 dB below the signal). This would also be sufficient for short signalling tones (e.g. DTMF, SEL5 or 1750 Hz for repeater access). But in case of CTCSS which is transmitted in a constant manner below the actual speech signal, the amount of (quantisation) noise and distortions generated by a 2-Bit-DAC is not acceptable.

Also CTCSS requires a reduction in deviation compared to the other signalling tones. The correct deviation for a CTCSS tone is about 1/10 of the maximum deviation of the speech signal, which directly leads to a CTCSS modulation voltage which is at 1/10 of the maximum signal voltage (-20 dB compared to full-scale).

Signal amplitude can only be adjusted for all kinds of generated signals using the "DATA MOD" pot. An attenuation on the digital side would only reduce the bad signal-to-noise-ratio further. 20 dB of attenuation would not be possible at all(given 12 dB SNR in the first place). 

The following two hardware modifications address these issues.

Both these modifications are mandatory to use CTCSS with model EVA5 by providing the required signal attenuation and quality.

This modification enhances the quality of the generated audio signal.

The software requires this modification to be installed, if audio signalling (1750 Hz, DTMF, CTCSS) is used. Without this modification, the generated audio signal will be significantly distorted in amplitude (however, the fundamental frequency component will not be affected).

This modification adds a electronically switchable low-pass-filter to the path for the generated audio signals. If active, the filter reduces the cut-off frequency of the path to about 300 Hz and adds a general attenuation of about 13 dB (the microphone audio path will not be affected).

This modification enables direct access to the MC micros modulator and demodulator, thus allowing FSK data transmission. Originally conceived for 9k6 packet-radio, the modification has proven itself useful for D-Star and paging (POCSAG).

(The description of the mod will follow soon)