Changes

= Audio Filters =

= Audio Filters =

The release of AllStarLink 1.01 software package contains a feature pioneered by Jeff DePolo, WN3A.  It is a set of audio filters for amateur radio applications.   

The release of PTTLink 1.01 software package contains a feature pioneered by Jeff DePolo, WN3A.  It is a set of audio filters for amateur radio applications.   

The default transmit and receive filters were designed around commercial standards. However, because amateur radio operators have the resources to use wider bandwidths, especially on UHF, they may prefer other filter types in order to provide higher audio quality. Over the years there have been other fixes for the [[Usbradio.conf|usbradio]] channel driver minimizing the distortions.  The results below proves the distortions are considerably lower than what a typical radio/repeater will add into the audio chain (.8% for the application vs. 3 to 5% or more for the radio).  Contrary to what you may have read elsewhere, the dynamic range of this application, when properly deployed, is far beyond the capabilities of wide-band FM radio.

The default transmit and receive filters were designed around commercial standards. However, because amateur radio operators have the resources to use wider bandwidths, especially on UHF, they may prefer other filter types in order to provide higher audio quality. Over the years there have been other fixes for the [[Usbradio.conf|usbradio]] channel driver minimizing the distortions.  The results below proves the distortions are considerably lower than what a typical radio/repeater will add into the audio chain (.8% for the application vs. 3 to 5% or more for the radio).  Contrary to what you may have read elsewhere, the dynamic range of this application, when properly deployed, is far beyond the capabilities of wide-band FM radio.

The following settings can be optionally used in a USB radio interface context in "[[Usbradio.conf|/etc/asterisk/usbradio.conf]]".  If these keyword and value pairs are not present, their values default to zero (0).  These alternate filters should be used with care.  Note that additional computing power and radio adjustments may be necessary depending on your application.  They have been extensively tested and work fine even with the Raspberry Pi3.  Unlike other distributions, we encourage the use of the usbradio channel driver (chan_usbradio), or what we commonly call "Full DSP" in AllStarLink 1.01.  This feature will also work in simpleusb, if that's what you prefer.  

The following settings can be optionally used in a USB radio interface context in "[[Usbradio.conf|/etc/asterisk/usbradio.conf]]".  If these keyword and value pairs are not present, their values default to zero (0).  These alternate filters should be used with care.  Note that additional computing power and radio adjustments may be necessary depending on your application.  They have been extensively tested and work fine even with the Raspberry Pi3.  Unlike other distributions, we encourage the use of the usbradio channel driver (chan_usbradio), or what we commonly call "Full DSP" in PTTLink 1.01.  This feature will also work in simpleusb, if that's what you prefer.  

This feature is immediately available in new AllStarLink installations and updates.  

This feature is immediately available in new PTTLink installations and updates.  

=== Receiver Audio Low Pass Filter Options: ===

=== Receiver Audio Low Pass Filter Options: ===

This will make a marked improvement in frequency response, both as far as local repeat audio goes as well as through the network.  With the default (zero) filters, frequency response is pretty restricted.  

This will make a marked improvement in frequency response, both as far as local repeat audio goes as well as through the network.  With the default (zero) filters, frequency response is pretty restricted.  

Here's a brief synopsis of what the frequency response from Rx to Tx is like with the DEFAULT (zero) filters that everybody is currently using '''(refer to page 2 of the PDF).'''

Here's a brief synopsis of what the frequency response from Rx to Tx is like with the DEFAULT (zero) filters that everybody is currently using

Ptt filter 2.png

So, you can see that the low-end suffers quite a bit, as it's almost 8 dB down at 300 Hz.  It also has overshoots in excess of 1 dB centered at 425 and 675 Hz.  The high end falls off quite sharply as well.

So, you can see that the low-end suffers quite a bit, as it's almost 8 dB down at 300 Hz.  It also has overshoots in excess of 1 dB centered at 425 and 675 Hz.  The high end falls off quite sharply as well.

By switching to all 1's for the filters, the response and flatness if greatly improved '''(refer to page 1 of the PDF).'''

By switching to all 1's for the filters, the response and flatness if greatly improved  

Notice the improvement at the low end and high end.  The frequency response is flat to within ± 1 dB from 286 Hz to 2.77 kHz.  Regardless of what low-pass filtering you have (or don't have) in your transmitter, nor what your channel spacing is (15 kHz versus 20/25), you should have no problems as far as excessive occupied bandwidth using all 1's for the filters.  HOWEVER (and here's a big however), the CM108, even when not being sent data by the channel driver, has about 350 uV RMS of residual wideband noise, with energy primarily falling at harmonics of the sample rate (48 kHz).  350 uV is about 71 dB below the maximum output level of the CM108 best-case, but the ratio may be much worse than that depending on how low you have txvoice set (i.e. it may only end up being 30 or 40 dB down if your Tx has a sensitive input and you have txvoice set accordingly low).  As such, we strongly DO NOT recommend feeding a CM108/119 output directly to the modulator without low-pass filtering in the exciter, or optionally in the radio adapter.  The LPF doesn't need to be anywhere near as aggressive as it would be in a stock radio of course, but it should sufficiently attenuate everything in the ultrasonic range.  As always, you should proof your contraptions with a spectrum analyzer before sticking them on a mountaintop!   

 

Notice the improvement at the low end and high end.   

 

The frequency response is flat to within ± 1 dB from 286 Hz to 2.77 kHz.  Regardless of what low-pass filtering you have (or don't have) in your transmitter, nor what your channel spacing is (15 kHz versus 20/25), you should have no problems as far as excessive occupied bandwidth using all 1's for the filters.  HOWEVER (and here's a big however), the CM108, even when not being sent data by the channel driver, has about 350 uV RMS of residual wideband noise, with energy primarily falling at harmonics of the sample rate (48 kHz).  350 uV is about 71 dB below the maximum output level of the CM108 best-case, but the ratio may be much worse than that depending on how low you have txvoice set (i.e. it may only end up being 30 or 40 dB down if your Tx has a sensitive input and you have txvoice set accordingly low).  As such, we strongly DO NOT recommend feeding a CM108/119 output directly to the modulator without low-pass filtering in the exciter, or optionally in the radio adapter.  The LPF doesn't need to be anywhere near as aggressive as it would be in a stock radio of course, but it should sufficiently attenuate everything in the ultrasonic range.  As always, you should proof your contraptions with a spectrum analyzer before sticking them on a mountaintop!   

Deviations from all filters being set to 1 follow:

Deviations from all filters being set to 1 follow:

In summary, we believe if you switch all of the filters to 1 you'll be very happy with the improvement in quality and no further tweaking will be necessary.  Our goal was to make the "1" filters - plug and play.  The improvement will obviously be greatest when both the repeater you're on as well as the repeater of the user(s) you're talking to have the updated filter code, and have taken advantage of these filters.

In summary, we believe if you switch all of the filters to 1 you'll be very happy with the improvement in quality and no further tweaking will be necessary.  Our goal was to make the "1" filters - plug and play.  The improvement will obviously be greatest when both the repeater you're on as well as the repeater of the user(s) you're talking to have the updated filter code, and have taken advantage of these filters.