Discussion on two methods of eliminating sidetone

In an intercom system where analog audio transmission and reception signals share a channel, in order to reduce the impact of sidetone on the call quality, all intercom equipment needs to add a sidetone elimination circuit. On the one hand, the sidetone elimination circuit allows the audio transmission signal to appear on the transmission line at a certain ratio, and on the other hand, the signal obtained by the local audio receiving circuit is small enough so that the speaker does not hear his own voice through his own speaker. At the same time, the receiver's sidetone elimination circuit allows the voltage on the transmission line to appear at a certain ratio at the input end of the audio receiving circuit, so that the receiver's speaker plays the sound.

The methods for eliminating sidetone usually include the transformer method, bridge balancing method and phase cancellation method. The transformer method was once widely used in dial telephones, but has now been eliminated. The bridge balancing method is widely used in key telephones, and the phase cancellation method is more common in wireless intercoms and building intercom systems. The phase cancellation method has a variety of circuit forms, and it can be found that these circuits are mixed and errors often occur. Therefore, it is particularly necessary to systematically explain the principle of the phase cancellation method and its circuit form. In addition, the bridge balancing method can be regarded as a special form of the phase cancellation method, which will be discussed in another article.

1 Use addition to achieve sidetone cancellation

1.1 Principle

In the series voltage divider circuit shown in Figure 1, R1 and R2 are pure resistors, v1 and v2 are input voltages, and vo is the output voltage. According to the superposition theorem:

 

Let vo = 0, then v1R2 + v2R1 = 0, that is:

 

In particular, when R1=R2, v1=-v2.

From formula (1), it can be seen that in order to make vo = 0, v1, v2 must meet two conditions:

(1) The phase of each frequency component is opposite;

(2) The amplitude of each frequency component is in a certain proportion and the proportion is the same.

 

1.2 Circuit form (I)

The audio input and output circuits of the two parties are generally the same. Figure 2 shows the sidetone elimination circuit of the two-party intercom, divided by a dotted line. The sum of R10 and R11 of Party A is much larger than R6, so the branch formed by R10 and R11 can be regarded as an open circuit. Similarly, the branch formed by R21 and R22 of Party B can also be regarded as an open circuit. For the AC signal output by U1, the DC blocking capacitors C2 and C10 at both ends of the transmission line can be regarded as a short circuit, and the output end of the operational amplifier U3 can be regarded as AC grounding, so R6 and R17 form a simple series relationship. Ignoring the loss of the transmission line, assume that the output of U1 is 1, which drops to 0.5 after passing through R6. Assume that the amplification factor of U2 is -A (the actual amplification factor of U2 in the figure is a positive value, but it is a negative value relative to U1). According to formula (1), 0.5R11=A×R10. Due to the symmetry of the circuits of Party A and Party B, Party B's R21=R10 and R22=R11. On Party B, the voltage obtained on the right side of R17 is 0.5, the output of the operational amplifier U4 can be regarded as AC grounding, and the emitter follower can be regarded as an open circuit, so the voltage obtained at the input of the emitter follower is:

 

When 2A is much greater than 1, the voltage obtained at the input of the receiver amplifier is close to 0.5. It should be noted that if multiple identical devices are connected in parallel on the audio transmission line, the voltage obtained at the input of the emitter follower will be smaller.

1.2 Circuit form (II)

FIG3 is a block diagram of a sidetone cancellation circuit provided in a document, and the principle of its application is the same as that described in FIG1 .

The circuits of Party A and Party B are still symmetrical, and R1, R4, R2 are equal to R5, R3, and R6. Although the literature does not give specific circuit parameters, it is easy to derive the voltage obtained by the receiving amplifier based on formula (1). Assuming that the output of U1 is 1, the input resistance of U2 and U3 is large enough, and ignoring the signal loss of the transmission line, the voltage at points b and d in the figure is 0.5. Assuming that the amplification factor of U2 and U3 is -A, it is known from formula (1) that 0.5A×R2=R3. At this time, the voltage obtained at the input end of Party B's amplifier is:

 

When A is much larger than 2, the voltage obtained at the input of the receiving amplifier is close to -1. If multiple identical devices are connected in parallel on the audio transmission line, the voltage obtained at the input of the amplifier will also be smaller.

1.3 Circuit form (III)

The circuit shown in Figure 4 is suitable for low-cost applications. The signals at the emitter and collector of the transistor are inverted, and the function of a transistor is equivalent to U1 and U2 in Figure 2. The bias circuit of the transistor is not drawn in the figure, and C1 and C2 isolate the DC component from the transmission line. It should be pointed out that if the adjustable resistor P1 is large enough, so that the bias effect on the transistor is small enough, C2 can be removed, and the adjustable resistor P1 is directly connected in parallel with the c and e poles of the transistor.

 

2. Use subtraction to eliminate sidetone

2.1 Principle

Figure 5 shows a circuit for subtracting two voltages vS1 and vS2. Under ideal op amp conditions, the output voltage can be obtained by using the "virtual short" and "virtual break" phenomena:

If the resistance value is selected to satisfy the relationship Rt/R1=R3/R2, the output voltage can be simplified to:

vo=Rf／R1(vS2-vS1) (5)

In particular, when vS1=vS2, vo=0.

 

It should be noted that there is a common-mode voltage at the two input terminals of the op amp, and an integrated op amp with relatively high common-mode rejection must be selected to ensure a certain level of calculation accuracy.

2.2 Circuit form

The audio circuit of a symmetrical two-party intercom is shown in FIG6 , with the dashed line as the boundary.

 

The in-phase bias circuit of op amps U1 and U2 can be referred to Figure 2, which is not drawn here. For AC signals, the capacitors C3 and C5 at both ends of the transmission line can be regarded as short circuits. When examining Party A's sidetone elimination circuit, the output end of op amp U3 can be regarded as AC grounding, and R15 is equivalent to R3 in Figure 5. Similarly, for Party B's sidetone elimination circuit, R7 is equivalent to R3 in Figure 5.

Take the case of audio signal sent by Party A, assuming that the output of U1 is 1, then the output of U2 is almost 0, achieving the purpose of eliminating sidetone. Ignoring the loss of audio signal on the transmission line, from the virtual disconnection phenomenon of the ideal operational amplifier, it can be seen that the voltage of the in-phase terminal of U4 is 0.5. At this time, the connection relationship between U4 and the surrounding resistors is shown in Figure 7. It is easy to obtain that the output of U4 is 1, and Party B has received the audio signal of Party A.

 

It should be noted that in the case of multi-party calls, for each additional identical device connected to the audio transmission line, the Rf of the op amp needs to be adjusted accordingly to satisfy the relationship Rf/R1=R3/R2.

The sidetone elimination function can be realized by adding the anti-phase signals and subtracting the in-phase signals. Because the addition and subtraction operations of the signals are used, the audio output signal of the sidetone elimination circuit is generally not suitable for isolation by transformers, because the isolation transformer and the DC blocking capacitor of the operational amplifier will often significantly change the phase of the party participating in the operation, and the sidetone elimination effect will be greatly reduced. In practical applications, it is best to add low-pass or high-pass filter circuits to the input and output operational amplifiers to limit the bandwidth of the input and output signals. Two resistors can be connected in series at R11 in Figure 2 to fine-tune the resistance value. When a sidetone elimination circuit is selected, the circuit parameters can be optimized with the help of simulation software to obtain the most suitable sidetone elimination effect.

After adding the sidetone cancellation circuit to the intercom equipment, in addition to saving an audio transmission line, it also facilitates multi-party calls by simply connecting the audio signal lines of each party in parallel.