[2658] How BBDs Sample Audio

Date: February 2nd, 2020 | Comments : [1] | Categories: DIY.

“An analog signal is one that is continuous in both amplitude and time.”

A bucket brigade device (BBD) does not render audio continuuosly in either time or amplitude. We can take a look at the inner working of the BBD chip to see why this is true.

The image below illustrates the input sampling circuit* of a BBD. If we start at the rising edge of the clock pulse, the Q1 mosfet turns on when the clock voltage is equal or higher than the threshold of the mosfet. Since the mosfet is now low resistance, the audio signal can pass from the input to the C1 capacitor, and will start to charge it.

The mosfet remains on during the entire time period T1 and the input voltage can vary during the period until we reach the falling edge of the clock pulse. When the clock pulse transitions from high to low and goes below the threshold voltage, the mosfet will turn OFF and go into a very high impedance state. The voltage level on the capacitor at that instant is locked into capacitor C1. This is actually when the sampling takes place. Only the voltage that is on C1 at that point is saved and the rest of the signal during the previous T1 period is discarded. No further sampling takes place until the next falling edge of the clock signal.

During the T2 interval, the charge on C1 is transferred into the capacitor of the first delay stage in the chip. No audio is going through Q1 during the T2 interval.

The sampling capacitor C1 has the ability to store an amplitude value of (theoretically) infinite precision, but it does this at only one point per clock cycle. In the time that occurs between falling edges of the clock, the amplitude of the input audio can go up or down, and so will the charge on the sampling capacitor C1. Only the falling edge of the clock will freeze the value into the capacitor. All of the variations in amplitude that have happened during the entire clock cycle are lost. If only one value of amplitude is retained for each clock cycle, it cannot be continuous and is not analog.

The voltage stored in each capacitor down the delay line is only a single point in a 2-dimensional matrix of amplitude and time. There is no continuous data stored for either range.

Let’s say that the rising edge of the clock is at time T0 and the falling edge is at T1 (where the sample will be taken). What is the value of the analog signal at T0.5? We do not know because it was not stored. What is the amplitude value at T0.25 or T0.75? Again, unknown.

While the storage medium (capacitor C1) has the ability to store an analog voltage of infinite accuracy, it only stores its precise samples once per cycle and the rest of the continuous audio voltage is ignored.

You can read my previous post called BBDs Are Not Analog.


From the University of Texas. http://users.ece.utexas.edu/~valvano/Volume1/E-Book/C13_DACSound.htm

*The sampling circuit and the timing of the data input came from the datasheet of the SAD1024 BBD chip.

 

One Response to “How BBDs Sample Audio”

[787224] admin Says: 11:39 am, February 3rd, 2020

“An analog signal is one that is continuous in both amplitude and time. Neglecting quantum physics, most signals in the world exist as continuous functions of time in an analog fashion (e.g., voltage, current, position, angle, speed, force, pressure, temperature, and flow etc.) In other words, the signal has amplitude that can vary over time, but the value cannot instantaneously change. To represent a signal in the digital domain we must approximate it in two ways: amplitude quantizing and time quantizing (or Sampling).” – Dr. Jonathan Valvano and Dr. Ramesh Yerraballi, University of Texas


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