All, I found a simple adjustment inside the original DX7s that reduces the white noise (hiss) significantly to an acceptable level. Google search for "DX7 Noise Reduction". It is on my YouTube channel in my DX7 playlist.
I am also studying the DX7 schematic and have read the DX7 user's manual and service manual. I am creating a video on how the DX7 works from the DAC to the Output and Headphone jacks. I have a good handle on the circuitry and theory of operation, I just need to make some slides. The DX7 has an 8 level DAC reference voltage (Vref) set by the CPU. Because the DAC is only 12 bits, very soft tones may be down to only 1 bit in sampling, this would cause very low amplitude signals to be tiny square waves which at very high frequencies sound like a hiss (white noise). It appears the CPU can set the VRef to 1 of 8 different levels at any instant. This makes sense, if the VRef is set low, the 12 bits of the DAC input can create very low signals with very fine resolution, and if the signal is strong, the Vref can be set higher to get the full dynamic range of 12 bits from 0V to 5V out of the DAC. The problem seems to be that this Vref can change sample to sample which is about 5MHz, the ramps of these changes and from the samples themselves is the source of much of this noise. The DX7 has 6 FM Synthesis Operators arranged in any one of 32 Algorithms at any given time. The DX7 can also apply the Algorithm to up to 16 keys (notes) at a time. So, one would think there are 96 Operators (6 X 16) in the DX7.
If fact there is only ONE Operator in the DX7! It is multiplexed up to 96 times to create a 16 note voice at any instant with a 45KHz Nyquist sample frequency. This multiplexing saves a lot of hardware, but requires the DAC to receive updates at about 5MHz. 5MHz is above human hearing, but the "edges" of clocking creates a wide band of noise that reaches down into the hearing range. This is what we hear as the hiss. It sounds like white noise, but it may have some pink noise aspects as well because the final stage Low Pass Filter can be adjusted to a cutoff point that is lower than the factory setpoint, thus cutting off the high end noise but leaving the high frequency quality of the notes intact. In the DX7-IID, a 15 bit DAC was used and the Vref is constant (less noisy). That DAC also has a single ended analog output, where the original DX7 had differential analog outputs and an external Operational Amp to convert it to single ended (this extra stage of circuitry before all the Low Pass Filter amps also contributes to the noise). All of my DX7 pots seem to be right smack in the mid position. I have a sneaking suspicion that Yamaha didn't even attempt to adjust them to keep up with production. I bought mine in 1984 and I had to wait over 6 months to receive it on back order. They were really backed up with orders as the DX7 introduced the world to digital synthesizers and thus they were very popular - more than expected.
I am also studying the DX7 schematic and have read the DX7 user's manual and service manual. I am creating a video on how the DX7 works from the DAC to the Output and Headphone jacks. I have a good handle on the circuitry and theory of operation, I just need to make some slides. The DX7 has an 8 level DAC reference voltage (Vref) set by the CPU. Because the DAC is only 12 bits, very soft tones may be down to only 1 bit in sampling, this would cause very low amplitude signals to be tiny square waves which at very high frequencies sound like a hiss (white noise). It appears the CPU can set the VRef to 1 of 8 different levels at any instant. This makes sense, if the VRef is set low, the 12 bits of the DAC input can create very low signals with very fine resolution, and if the signal is strong, the Vref can be set higher to get the full dynamic range of 12 bits from 0V to 5V out of the DAC. The problem seems to be that this Vref can change sample to sample which is about 5MHz, the ramps of these changes and from the samples themselves is the source of much of this noise. The DX7 has 6 FM Synthesis Operators arranged in any one of 32 Algorithms at any given time. The DX7 can also apply the Algorithm to up to 16 keys (notes) at a time. So, one would think there are 96 Operators (6 X 16) in the DX7.
If fact there is only ONE Operator in the DX7! It is multiplexed up to 96 times to create a 16 note voice at any instant with a 45KHz Nyquist sample frequency. This multiplexing saves a lot of hardware, but requires the DAC to receive updates at about 5MHz. 5MHz is above human hearing, but the "edges" of clocking creates a wide band of noise that reaches down into the hearing range. This is what we hear as the hiss. It sounds like white noise, but it may have some pink noise aspects as well because the final stage Low Pass Filter can be adjusted to a cutoff point that is lower than the factory setpoint, thus cutting off the high end noise but leaving the high frequency quality of the notes intact. In the DX7-IID, a 15 bit DAC was used and the Vref is constant (less noisy). That DAC also has a single ended analog output, where the original DX7 had differential analog outputs and an external Operational Amp to convert it to single ended (this extra stage of circuitry before all the Low Pass Filter amps also contributes to the noise). All of my DX7 pots seem to be right smack in the mid position. I have a sneaking suspicion that Yamaha didn't even attempt to adjust them to keep up with production. I bought mine in 1984 and I had to wait over 6 months to receive it on back order. They were really backed up with orders as the DX7 introduced the world to digital synthesizers and thus they were very popular - more than expected.
Statistics: Posted by Romsk — Sun Dec 03, 2023 4:20 am