Changes

* txhpf=2;    120 Hz cutoff for special applications requiring additional bass response in transmitted audio.  Not recommended due to the increased possibility of voice energy interfering with sub-audible signaling, but should work okay with very low CTCSS frequencies.  

* txhpf=2;    120 Hz cutoff for special applications requiring additional bass response in transmitted audio.  Not recommended due to the increased possibility of voice energy interfering with sub-audible signaling, but should work okay with very low CTCSS frequencies.  

Key points and specifications:

== Key points and specifications ==

# All filter combinations yield aliasing distortion/folding at or below -40 dBr.  Note that all testing was done with the usbradio channel driver using discriminator (flat) Rx audio, and with Tx audio preemphasis in software.  The measurements that follow *include* the low-pass filter inside the DMK URI in the transmit audio path, which is down about 3 dB at 3.5 kHz.  The frequency response is better with a modified fob or radio adapters with wider frequency response.  

# All filter combinations yield aliasing distortion/folding at or below -40 dBr.  Note that all testing was done with the usbradio channel driver using discriminator (flat) Rx audio, and with Tx audio preemphasis in software.  The measurements that follow *include* the low-pass filter inside the DMK URI in the transmit audio path, which is down about 3 dB at 3.5 kHz.  The frequency response is better with a modified fob or radio adapters with wider frequency response.  

# The variations in amplitude response within the passband as you switch filters are fairly minor.  A perfectionist would re-calibrate both rx and tx levels after switching filters in the conf files, but if you're not at the site with a service monitor, but you shouldn't be afraid to switch filters without recalibrating as the variation is less than 0.3 dB @ 1 kHz worst-case among all filter combinations.  Yeah, the filters were designed for unity gain, but there's a little ripple in the passband, plus the ulaw effects, so nothing's perfect, but it's close enough to perfect to not worry about.

# The variations in amplitude response within the passband as you switch filters are fairly minor.  A perfectionist would re-calibrate both rx and tx levels after switching filters in the conf files, but if you're not at the site with a service monitor, but you shouldn't be afraid to switch filters without recalibrating as the variation is less than 0.3 dB @ 1 kHz worst-case among all filter combinations.  Yeah, the filters were designed for unity gain, but there's a little ripple in the passband, plus the ulaw effects, so nothing's perfect, but it's close enough to perfect to not worry about.

# THD did not appreciably change as filters were varied, it tends to hover around 0.8% from 300 Hz to wherever the LPF's start to kick in.  At low amplitudes (either due to low audio levels or filter skirt attenuation), obviously distortion, as a ratioed value, appears to rise due to quantization noise, but that's unavoidable in our little 8 bit ulaw corner of the world.  IMD was likewise acceptable across the range.  

# THD did not appreciably change as filters were varied, it tends to hover around 0.8% from 300 Hz to wherever the LPF's start to kick in.  At low amplitudes (either due to low audio levels or filter skirt attenuation), obviously distortion, as a ratioed value, appears to rise due to quantization noise, but that's unavoidable in our little 8 bit ulaw corner of the world.  IMD was likewise acceptable across the range.  

 

Start off with all of the filters (rxlpf, rxhpf, txlpf, txhpf) set to 1.   

Start off with all of the filters (rxlpf, rxhpf, txlpf, txhpf) set to 1.