Chapter 632 Chasing Gao Zhendong who is feeding
"According to my experience and some experimental calculations, to achieve a satisfactory level of communication, the signal-to-noise ratio should be greater than 26dB. To achieve this effect, combined with the dynamic range of the voice signal up to 40dB, we can calculate that if a uniform For quantization and linear encoding, the code length will reach 11 bits to meet the requirements! ”
11 bits may not seem like too much, but in a time division multiplexing system, this is unacceptable. The allocated time slots are inherently short, and it is difficult to transmit such a long code.
And for the encoding and decoding circuit, the code length of 11 bits means that there are 2048 levels! It is really difficult for current circuits.
At this time, everyone who thought it was simple just now realized the seriousness of the problem. The theory seemed very simple, but once it was put into practice, it was discovered that there was a reason why we had been clueless in this regard.
The theory is simple, but the actual requirements are very troublesome.
"Eh? This is troublesome..." Someone bit the pen.
If you want to separate 2048 levels, AD and DA circuits with this kind of precision are a bit too high for now. Even decades later, there was a time when a 10-bit AD could be considered High-precision AD devices generally use 8-bit AD.
At this moment, even the comrades of the Military Communications Institute who did not consider time division multiplexing for the time being and therefore did not have high requirements for the code length, could not laugh anymore. Let alone whether the code length is not long, let’s talk about whether you can get this code first!
After thinking for a long time, I couldn't figure it out: "General Engineer Gao, how to solve this problem?"
Gao Zhendong smiled and said: "I just mentioned that under the condition of uniform quantization, the situation is like this..."
One comrade understood a little bit: "General Engineer Gao, you mean that non-uniform quantification can solve the problem?"
Gao Zhendong nodded.
But how to do this non-uniform quantization? In other words, what are the benefits of non-uniform quantization? I don’t know how this benefit comes from. If I don’t know this, I naturally can’t think about how to operate this method.
Gao Zhendong said: "In fact, the signal-to-noise ratio is segmented in the signal. For small signals, the signal-to-noise ratio will be low with the same noise, but for the same noise, for large signals, the signal-to-noise ratio will be lower. The ratio is much greater, you might as well think in this direction.”
"Signal-to-noise ratio...the same noise...yes, whether it is white noise or quantization noise, it basically does not deviate too much within a range...non-uniform quantization...General Engineer Gao, what do you mean? Yes, for the small signal segment, give it more grading, and for the large signal segment, give it less grading. Although it is uneven, according to the theoretical calculation you just made, the quantization noise signal-to-noise ratio of the small signal segment will increase. , the big signal is not affected much, just ignore it!”
A comrade from the Military Communications Office thought for a long time, seemed to have figured out the way, and expressed his thoughts very excitedly.
His words were very difficult to pronounce and difficult to understand, but most of the comrades still understood them.
Before Gao Zhendong could speak, other comrades shouted.
"Yes, we will take care of whichever section is in worse condition."
"This can be regarded as taking care of the old, weak, sick and disabled. It seems that doing good things to others really makes sense."
"This is an incredible idea! I really can't think of it."
"..."
Gao Zhendong nodded: "Well, that's roughly what it means."
The comrade thought for a while, but was a little dumbfounded. This would be too difficult for the AD/DA circuit. With uneven sampling and quantization encoding, the AD/DA circuit has to determine which segment the signal is in, and then switch to different quantization. Weighted, this circuit doesn't sound much simpler than an 11-bit AD/DA.
Moreover, this kind of non-uniformity must not be simply divided into two segments, but into many segments. Just thinking about this AD circuit makes my scalp numb!
Seeing everyone worrying about this difficult AD/DA circuit, Gao Zhendong smiled.
"Comrades, let's change our thinking. We should process the signal first, and then let the digital-to-analog circuit perform the conversion."
"Handle it? How to deal with it?"
"Before the signal enters the AD/DA circuit, we first enter a signal preprocessing circuit. This preprocessing circuit is at the AD end. We call it a compression circuit. Its function is to change the signal. Small signals are amplified and large signals The input and output characteristic curve remains unchanged, and its input and output characteristic curve is a convex curve. In this way, when combined with ordinary AD, the signal will naturally be non-uniformly quantized. ”
Gao Zhendong drew the picture on the blackboard. Following the lines and Gao Zhendong's explanation, everyone finally understood what it was.
Brilliant idea!
"Wonderful! The AD is liberated from non-uniform work and the non-uniform preprocessing is handed over to the compression circuit, so that the complexity of the circuit does not increase much!"
"This is the same as non-uniform quantization. It breaks out of the original framework and finds new ways to achieve the goal cleverly!"
"Then when it comes to voice reproduction, before entering DA, we must enter the expansion circuit..."
"What do you think? You should enter the expansion circuit after entering the DA. Otherwise, it will still be a digital signal. How can you expand it?"
"Hey, yes, yes, I was stupid..."
"..." While everyone was discussing, they were so impressed by Gao Zhendong's fantastic ideas. No wonder Chief Engineer Gao asked just now, "Isn't it simple?" It turns out that the real trouble lies in these places. But these troubles were solved beautifully by him and the price was not high.
Seeing that they all understood it almost, Gao Zhendong said: "You can use the formula and rules we used to calculate the signal-to-noise ratio just now to calculate and see how many bits are needed for non-uniform quantization to achieve the effect of uniform quantization of 11 bits."
After hearing this, all the comrades grabbed paper and pen, and started their own calculations based on the formulas and calculation processes used by Gao Zhendong to calculate uniform quantification on the blackboard.
The calculation is very slow, and there are connections and differences between the two. Some comrades who have just come into contact with this aspect have not even found the direction, and they have to slowly explore the substitutions and transformations between formulas.
Finally, one comrade shouted out.
"8 bits! Damn it! 8 bits is enough!"
Although he was a mature and prudent old comrade, he still couldn't help but curse. This improvement was really too astonishing.
The rest of the comrades were shocked, not because he calculated quickly, but actually calculated slowly enough. The reason for surprise was that this performance improvement was too exaggerated.
The difference between 8-bit and 11-bit is not just the difference between 8 and 11, it is 8 times the difference!
This means that the original 2048 levels to be processed are now reduced to 256 levels, which is an order of magnitude lower!
In this way, whether it is AD or DA, it will be much easier to do it.
"General Engineer Gao, did I calculate it correctly?" This comrade himself didn't believe his results. Did he calculate it wrong? asked Gao Zhendong with a hint of anxiety.
Gao Zhendong smiled and said: "The calculation is correct, this is the data! The non-uniform quantization is indeed 8 bits."
This is the charm of mathematics. This processing method is actually a mathematical method rather than a physical method. In fact, the entire digital signal processing system is basically the charm of mathematics.
The comrades of the Military Communications Institute are full of confidence! This time they dare to guarantee that they will go back and turn the next generation radio station into a complete digital radio station!
The most difficult thing to break through is the principle. Once the principle is broken through, you can find ways to implement it.
But since Gao Zhendong has said so much, he will definitely not let them waste too much time. This is just the principle of encoding. As for the specific encoding, they don't know yet.
Gao Zhendong said: "Comrades, now turn to page XX, the A-law characteristic 13 polyline code."
This is actually the non-uniform quantization coding method that my country later adopted on PCM equipment. Gao Zhendong figured it out early.
The compression curve is divided into two sections, both of which are logarithmic functions with A as a constant. This constant A determines the shape and properties of the curve, hence the name A-law.
Our country’s A=87.6.
Since a strictly consistent compression curve is difficult to achieve on a circuit, 13 segments of polyline are used to approximately replace this curve.
Everyone was happy when they heard this. Chief Engineer Gao is a good man who will go to the end and send Buddha to the West. Even the compression curve has been selected. They are sure that this compression curve is definitely the most suitable within a certain period of time. A compression curve.
The knowledge of A-law compression coding is not abstract, but the length is slightly longer. Gao Zhendong spoke very quickly, and the comrades also understood it deeply. They dared to pat their chests. With this thing, don’t go back and design a set of speech codec circuits. So easy.
But they obviously underestimated Gao Zhendong's determination to chase and feed him.
"Comrades, next, let's discuss what in my personal opinion is the most suitable PCM codec circuit at present-successive feedback encoder and weighted network decoder!"
The comrades were so happy that they couldn’t open their mouths from ear to ear. Hey, look, come to Mr. Gao’s place. Not only are you full, but you also get takeaways!
This is obviously beyond the scope of what Gao Zhendong said at the beginning, which was to call everyone together to teach the principles, and to implement it directly. Although it should be impossible for Chief Engineer Gao to come up with the specific circuit, one look at him The encoder block diagram in the information does not mean that the food is poured directly into the stomach through the tube, but at least the food is prepared and served on the table.
It seems that Chief Engineer Gao was afraid that his comrades would have difficulty digesting it and waste time, so he just started it himself!
These two things took a long time, because they were both very practical. Gao Zhendong broke apart all the contents of the two circuits one by one and taught them to the comrades carefully.
His idea was simple. Since he had this knowledge, he would teach it all to his comrades at once to reduce the possibility of them taking detours and save as much time as possible.
For Gao Zhendong and his ideas, time is everything.
(End of chapter)