How to do HiFi AM on the Kenwood TS-850S/AT and DSP-100

Written by:

Bill Paul N1GPT

The TS-850S/AT and DSP-100 combo is well known for being able to produce outstanding HiFi SSB audio. Using a couple of special power-on menu options, it's possible to achieve up to 6Khz of audio frequency response. However, due to a design quirk, once you set the LPF option to 'off' to open up the high end response, the DSP will no longer generate a carrier when the radio is set for AM mode. This prevents the operator from achieving Hi-Fi sound on AM.

It turns though that you can make the radio transmit in HiFi mode in AM, if you trick it. I've discovered that if you begin transmitting in AM, and then switch to either LSB or USB mode and then back to AM mode again. (Using FM instead of LSB or USB also works. CW does not.)

The only problem is that you have to do this every time you transmit, which makes operation clumsy. Fortunately, I've found two ways to address this. One is to use an external computer assist program to shift modes automatically when the user engages the transmitter. The other is to patch the firmware for the main CPU so that it works around the DSP's glitchy behavior internally. This page provides links to both solutions.

The software-only solution has the advantage that no modifications to the radio are required at all: if you have your radio connected to a computer already, you just need to run the program and leave it going in the background. However it has a few drawbacks: you can't use any other CAT software while the program is running, and it's a little slow: it may take a half second or so after you press the transmit button on the radio before the program senses it and swaps modes.

The firmware patch solution is more seamless however it's not quite perfect. On the TS-950SDX, the DSP unit is built-in, and the main computer in the radio has more fine-graned control of it. This allowed for a more elegant, precision workaround that works around the AM problem perfectly. However with the TS-850, the DSP unit is external and is controlled by a separate internal processor which, sadly, can't have its firmware upgraded. The firmware mod for the TS-850's firmware therefore has some limitations. It was observed that when using it in the Mid-Fi case (where AM already works), toggling the mode would result in a brief burst of full output power, which is undesirable. A similar behavior was observed when the VOX was enabled. Consequently, the fix can be toggled on and off using a power-on hot key so that it does not adversely affect non-HiFi operation, and it will not operate while the VOX key is pressed. It also requires making a new firmware EPROM and opening up the radio to install it.

To download the CAT utility, click here:

Kenwood TS-850/TS-950SDX AM DSP CAT utility

To download the modified firmware, click here:

Patched HiFi AM Kenwood TS-850S/AT firmware image