Understanding the YM 2151- YM 3012 Relationship

Hi FPGA lovers! In our conquest to find studio quality sound, we stumbled upon the YM 2151 FM Synth Chip which has been used by Yamaha from time immemorial in a number of their electronic keyboards and arcade games. The YM 2151 has a Digital output rather than an analog output and this makes it ideal for further signal processing using DSP. However due to the absence of an on chip DAC, the YM 2151 needs a suitable DAC chip that does not add substantial noise while analog reconstruction.

Today’s post is a detailed study of the YM 2151 and its coupling with YM 3012 DAC which are almost like inseparable siblings of the Yamaha family. The YM 2151 is a 24 pin IC which gives a 16 bit output.  The first 3 bits have no significant value, whereas the next 10 bits in order are the significant bits or mantissa and the last 3 bits are the exponents. The author has conducted a number of measures for a fixed value of supply voltage and has shared the results in his post.

The 3 parameters of concern are “d”, “n” and “m” which are the value of mantissa, exponent value and size of LSB respectively. The output voltage of the chip is given by a fixed relationship between these parameters where Vout= m*d + n.

From this result, the Verilog code for YM 3012 in the post starts making sense. The code has been designed around the fact that the base voltage is Vdd/2. However no explanation was given as to why this particular voltage was chosen. From this result, we deduce that Vdd/2 is the principle value because it supports maximum swing in positive and negative directions. This way the 10 bit resolution of the mantissa can be put to full use to produce the desired harmonic frequencies of high quality and clarity.

This is the reason why the YM 3012 DAC is best for this Synth chip, because it is essentially the second half of Yamaha’s FM Synth Chip design. The chip has been broken into two parts (the 2151 and 3012) to give the benefit of getting a 16 bit Digital output for further processing before reconstruction of the analog signal.

This makes the YM 2151 perfect for creating sound boards. The YM 2151 can be used in combination with a FPGA to recreate any themes. The FPGA adds a degree of flexibility in the chip’s output since the output of the YM 2151 is digital and can be easily processed by the FPGA.

By Jotego