CAT 1000 Audio Bandwidth Improvement

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Back to Home Modifying the CAT-1000 repeater controller for wider audio bandwidth
By Jay Farmer N8IPO and Kevin Custer W3KKC

Concept:
Modify the CAT-1000 for increased audio bandwidth when increased bandwidth is desired. Increased bandwidth may be necessary when using the controller in linking systems where cascading filtering effects will limit overall bandwidth of the system. This modification may also be necessary in single repeater systems where people can properly use available bandwidth. This is especially true in UHF systems where more bandwidth is available, and/or where Flat Audio Modification and elimination of the emphasis is desired.

The CAT-1000's bandwidth from the factory is approximately 300 cycles to 2.5 kHz and does not provide de-emphasis of the receiver audio. This bandwidth is fine for most systems, however when this controller is used in linking systems where the passing of PL tones or more occupied bandwidth is desired, the modifications below can be beneficial.

The low end response can be improved to allow PL tones to pass and the high end improved to allow crisper audio to be transferred. This is especially true in hub systems where series linking may occur. The approach was to increase the audio range of the CAT as much as possible. In other words pass all the audio that the receiver hears with no filtering in the CAT.

Because, after modification, there is no filtering in the CAT, you will let the transmitter do the filtering, or add brickwall filtering so the modulation index is preserved but not violated beyond acceptable use of bandwidth. Changing these components does not add de-emphasis to the receive audio path. Also, the changing of C10 and C9 may not be necessary in newer units because the value may already be 1.0 uF.

The modification of the motherboard as specified in this page is accepted by Computer Automation Technology.

Modification:
On the CAT Motherboard:
Change C10 & C9 from .1 uF. to 2.2 uF.
This increases the audio input low end response to 30 cycles.

Also on the motherboard change C19 & C16 from .0015 uF. to 220 pF.
This increases the high end response from 2.5 kHz. to 15.8 kHz. @ -3 dB.
The response will now be flat from 30 cycles to 13.0 kHz. @ -1 dB with 1 kHz. as reference.

If you are using this controller in a system employing "Flat" audio throughput, some additional modification to the response of the voice and tone generator circuitry along with the phone patch may be necessary for acceptable performance.
Preemphasis of the voice and tone synthesizer, and phone return circuits may be necessary. De-emphasis of the audio going to the DTMF decoder and phone line may be necessary.

NOTE: The modification of the Audio Delay Board and DVR is not accepted by CAT at this time.

On the CAT Audio delay board:
Change C5, C13, C6 & C15 from .1 uF. to 2.2 uF. to increase the low end response.
Change C16 & C17 from .0015 uF. to 220 pF. to increase the high end response.
However: Highend response will still be limited due to the sampling rate of the delay module.
With the delay board in place, the audio path response is limited to 30 cycles to 2.5 kHz.

On the DVR:
Change C9 & C11 from .1 uF. to 2.2 uF. to increase the low end response.
Change C8 from .001 uF. to 220 pf to increase the high end response.

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This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.