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216 Audio

Audio IF From U32603-1 U13701 DeCoder Stereo Decoder 5 (R) 7 (L) U13702 10 8 U11402 3 1 5 7 L Aux 1 L Aux 2 L Aux 3 L-DVD Crtl A From SysCon Crtl B U13101-7/8/9 Crtl C (L) (R) (L) VGA #2 Audio IN (R) 11 10 9 12 15 R Aux 1 R Aux 2 R Aux 3 R-DVD U26101 3 (L/M) (R)
N/C

U11401 (L) 12 15 14 13 5 1 11 10 9 15 16 17 29 (L) 7 U11301 3 5

U11901 (L) 11 4 U11900 (L) 7 PwrAmp (R) 6 3 (R) (L) IIC IIC Dat Clk Head Phone (R) Jack 1 (L) Variable Out

30

27

(L) L Ext Spk R Ext Spk

TVB U11800
(R)

VGA #1 Audio IN

12 11 13 8 19 18 7 26

(R)

U11902 (R) 1 11 3 3 U11301 (L) Fixed Out #1 (before SRS (R) & TVB) R L L R (L) (R) Sel. Stereo Out #2 (after SRS & TVB) PwrAmp 4

14 13 5 1 U11400 (R)

FAV

U11402 10 12 14 8

1 8

SRS Circuit

10
N/C

Figure 15-1, Audio Switching/Processing Block Diagram

Audio 217

Audio Switching & Processing Overview The audio input capabilities (see block diagram) of the MM101 include the normal NTSC off-air audio, three different Auxiliary inputs, a left & right DVD (YUV or Y, Pr, Pb) audio input, separate VGA#1 & VGA#2 audio inputs along with a convenient front panel audio input jack. The audio outputs include a Fixed (before SRS & TVB) & Variable (after SRS & TVB) volume controlled. External speaker outputs are also provided along with capabilities to disconnect the internal speakers. The audio processing and switching circuits of the MM101 contain two major processing ICs along with various switching and amplification integrated circuits. As shown in the upper left of the Figure 15-1, IC U13701 contains the Stereo/SAP decoders while all signal routing and switching is performed by discreet switching ICs, U11400 (R) and U11401 (L). These input switching ICs are controlled by the main microprocessor, U13101, via three individual control lines. The tone, volume and balance (TVB) processing is performed within IC U11800 and controlled by the System Control Microcomputer U13101 via the IIC bus. The TVB IC also contains switching output lines controlled over the IIC bus and used to control the input switching IC's. Switch ICs U11400 and U11401 are responsible for routing and switching the auxiliary, DVD audio signal and the front panel audio input signals. U11400 handles the Right while U11401 handles the Left audio signals. Selection of VGA#1 and VGA#2 audio signals is handled internally by the TVB IC, U11800. Selected audio (Fixed) exits the TVB IC at pins 8 & 19. The signals are applied to the SRS circuit for processing or input directly back to the TVB IC at pin 7 & 18. The external Variable output connections are made here. After re-entering the IC at pins 7 & 18 Tone, Volume and Balance are added to the signals. The audio is then applied to the power amplifiers U11901 & U11902. The audio from pins 26 and 27 is also applied to an amplifier (U11900) which provides the HiFi (variable level) output and provides signal to the front panel headphone jack. At this point one more connection is made to a stereo output jack mounted on the rear jack panel. This provides a variable line level output for connection to an amplified speaker system. The speaker switch selects between the internal speakers and the external speaker outputs.

218 Audio
Left In From TVB IC U11800-8/19 Right In 2 +6V
Part Of U11701

VCC +12V L-R 1 +L R26 R12 5 'B' 4 7

+6V

L-R 2 R9 3 R10

U11702
1

3

'A'
R24 6 R35
20K Part Of U11701

L

+R +6V 12

R14
Part Of U11701

+6V R23 R36
20K

(L-R)+(L+R)=2L

+6V +6V R31

(L+R)-(L-R)=2R

L+R -R

5 6

7

R

14 13

10

Part Of U11701

'D'
+6V 9

8 L+R R11

8 VCC +12V

'C'

3 SRS ENH(LO)/Norm(Hi) From SysCon U13101 SRS On(LO)/Off(Hi) Out to TVB IC U11800-7/18 Right Out Left Out 14 9 10/11 15

4

+6Vr

1 2 13 12 L

R Right

Left

Multi-Plexer U11702

Figure 15-2, SRS (Sound Retrieval System)

SRS (Sound Retrieval System) Circuit The SRS circuit (Figure 15-2) of the MM101 audio system is located on the I/O (input/output) circuit board. The inputs to the SRS circuit are Left & Right audio signals from TVB IC U11800. There are three modes of SRS operation, Off, Normal-On and Enhanced-On. Multiplexer IC U11702 is utilized to control and route the SRS processed audio signals. SRS can be thought of as an inserted processor because it is placed in the audio at the beginning of the variable audio path.

Audio 219

IC U11701 and its associated components along with amplifier U11702 make up the SRS circuit. The left audio signal is applied to pin 3 of U11701 and the right audio is applied to pin 13 of U11701. The +L output at pin 1 is summed with the -R output of pin 14 and applied to pin 5 (L-R). The +R and +L are summed together and applied to pin 10. The output at pin 8 is the L+R signal. The output at pin 7 is the L-R audio signal which is equalized and gain boosted according to SRS specifications relative to the L+R signal. L-R and L+R are summed together generating Left only which is applied to pin 3, the non-inverting input of power amplifier U11702. In order to generate the Right only signal, the L+R signal from pin 8 is applied to the non-inverting input at pin 5 while the L-R signal is applied to the inverting input (pin 6). The resultant output signal at pin 7 of U11702 is the Right only signal. The Left and Right output signals are reduced by 1/2 at the input of U11702. Therefore, while the sum and difference signals would normally result in a "2L" and "2R" output according to the formula, instead they result in a normal output level. On/Off control of the SRS circuit is provided by the system control microcomputer U13101. Two signals, SRS ENH/Norm and SRS On/Off are applied to the switching IC U11702 at pins 9, 10 & 11. Control pins 10 & 11 route the audio around the SRS circuit or allow it to be process by the SRS circuit. The SRS ENH/Norm signal at pin 9 controls the gain of B stage. To accomplish this pin 9 of U11702 is pulled Lo, closing the switch and applying +6V to R36. This effectively parallels R24 (20K) and R36 (20K) equaling 10K ohms. This increases the gain of the L-R stage (B) thus generating more L-R signal enhancing the spatial properties of the signal. The SRS feature in the MM101 has a moderate gain factor to enhance the stereo sources and widen the image somewhat. The L-R channel is defined as carrying the side information while the L+R channel carries the front sounds. Filtering also helps keep the midrange from being overly boosted. Both paths have filters to help restore directional clues by compensating for the ears directional frequency response.

220 Audio Compressor Operation The compressor function in the MM101 is implemented via software control of TVB IC U11800. This control is via the IIC bus directly to the TVB IC. The left and right audio channels are summed together and applied to pin 2 of IC U11500. The output at pin 1 is applied to a half wave rectifier which generates an average DC level of the audio signals. This voltage appears at the cathode of CR11502. The output at pin 7 of U11500 and diode CR11503 constitute a 5 volt clamp. The clamp is necessary to prevent the signal to the microprocessor A/D input from exceeding 5 volts, preventing damage to the microprocessor. The microprocessor input responds any time the A/D voltage rises above 0V. During compressor operation, the average DC level of the audio signal is applied to the Sound Logic A/D input of the system control microcomputer, U13101-39. The software of the microprocessor watches the DC level and at a predetermined level issues a Volume Down command to the TVB IC U11800 via the IIC bus. The response time of the compressor circuit is approximately 10 to 13 milliseconds. If the audio peaks rise above the predetermined level, the main volume level is reduced very quickly. Conversely, when the peak audio level falls to normal levels, the volume is slowly turned back up to normalize the sound level. In this way the overall dynamic range of the audio signal is reduced so audio output levels do not change dramatically from one program segment to another or during channel change, etc.

Figure 15-3, Audio Compressor

Compressor

Left Aud IN
(From Switch IC U11702-14/15)

2 3 +5V 5 +12V 6

1 U11500 7 CR11501 CR11502

Right Aud IN

To Audio TVB IC U11800 for Volume Control ata Clk D C 43 44 IIC Bus SysCon U13101

CR11503

14 5 IIC Bus Multplexer U13203

39 Sound Logic A-D (Compressor)

Audio 221
( Mute ) 12 11 U11901 Audio Out 4 L11902

Left
From TVB IC U11800-26/27

L
L Ext Spk R Ext Spk

Right
Spk Mute
from U11800-8)

11

4

L11904 CR8

R
CR7 CR9 10V To Audio TVB IC U11800 for Volume Control Clk 44 5 14 D C 43 IIC Bus Multplexer U13203 Data

12 ( Mute )

U11902 Audio Out

+5Vr

Q3 Mute SW 12 AVR AVR Q4 Switch

SysCon U13101

IIC Bus

Head Phone Mute
(from U11800-3)

Left 7 Q2 6 Right Q1

3 2 1 U11900 +12V VCC

Sel Left Out #1

Sel Right Out #1 +12Vr Mute Head Phone Jack

Figure 15-4, Audio Output/AVR/Headphone Circuits Audio Output Circuit The audio output power amps in the MM101 are somewhat unique in that they operate as Class D amplifiers. Most audio output amplifiers are Class A or Class B linear amplifiers. The best-case operating efficiency for standard linear Class A & Class B amplifiers is around 70%. The Class D amplifier used in the MM101 operates at approximately 95% efficiency. During operation U11901 and U11902 generates an internal square wave signal at approximately 150kHz. All the circuitry needed to generate this square wave is contained within the ICs. The frequency of this square wave is determined by the ICs external resistors and capacitors. The standard analog audio signal is input at pin 11 of the IC. This audio signal is used to PWM (Pulse Width Modulate) the 150kHz square wave. The amplified signal exits at pin 4 of the IC. The passive components consisting of L2, R25, C13, C33 & C15 (Left) make up a Low Pass Filter. Component values are selected so the filter passes the audio frequencies but does not pass the 150kHz signal.

222 Audio The left and right audio signal from the TVB (tone, volume & balance) IC U11800 pins 26 & 27 is capacitively coupled into the audio power amps U11901 and U11902 at pin 11. The output of the power amps (pin 4) are applied to the EXT SPKR switch. The amplified audio is routed to the internal or external speakers depending on the position of this switch. Due to power output limitations of the amplifiers U11901 and U11902, internal and external speakers can not be driven at the same time. The left and right signals from the TVB IC are also routed to the driver IC U11900. This IC drives the headphone jack and also supplies signal for the selected variable output jacks on the rear jack panel. The headphone jack is located on the front of the instrument for easy access. The main speaker mute control signal comes from the TVB IC U11800 at pin 8. The main Mute signal (active Lo) is applied to pin 12 of both power amplifiers. Another source for a main mute signal is from the headphone jack. When the headphones are plugged into the jack, ground is lifted from the base of Q3 causing it turn on. When Q3 turns on, pin 12 of the power amplifiers ICs are pulled Lo. The signal that mutes the headphones is provided by the TVB U11800 pin 3. Headphone mute signal is applied to the base of Q1 and Q2, turning them on. With Q1 and Q2 on, the left and right audio signals going into pins 6 & 7 of U11900 is grounded out. Whenever the headphone is muted the audio that is going to the selected variable output jacks is also muted. AVR (Automatic Volume Reduction) Automatic Volume Reduction lowers the audio input level to the output amplifiers U11901 and U11902 during high volume passages that might cause an over-voltage condition and damage speakers connected to the outputs. The AVR control signal is generated using the audio output from the power amplifiers. The left and right audio output signals are applied to rectifier diodes CR7 and CR8. These diodes generate a DC voltage that is representative of the volume level. The 10V zener diode CR9 provides the reference for the AVR circuit. The DC level on the cathode of zener CR9 goes up as the volume level goes up. When the voltage rises above 10VDC, the zener diode breaks over, Q4 turns on and pulls pin 12 of U13101 Lo. The AVR input to the system control microcomputer U13101 is pin 12. The AVR control line is polled periodically to check the status of the volume output level. Whenever the input to the System Control Microcomputer at pin 12 is Hi (Q4 Off), no action is required. When this pin is pulled Lo by Q4 turning On, the system control microcomputer outputs a volume down command via the IIC bus. This volume down command is routed through the IIC bus Multiplexer and is applied to the TVB IC U11800. The volume is automatically reduced until the sense input (pin 12) to the system control microcomputer goes Hi again. Then the volume stops decrementing and a timer is started. If the line goes Lo again, the timer is canceled and volume is again reduced until the sense line goes Hi. If the timer runs down and the sense line has stayed Hi, volume is slowly incremented back up to the customer setting. If the AVR sense line goes Lo again, incrementing stops and the decrementing loop is again entered. The consumer will notice a rapid decrease in volume followed by a longer ramp up to the original volume setting.

Audio 223

( Mute )

12 11

U11901 Audio Out 4 L11902

Left
From TVB IC U11800-26/27

L
L Ext Spk R Ext Spk

Right
Spk Mute
from U11800-8)

11

4

L11904 CR8

R
CR7 CR9 10V To Audio TVB IC U11800 for Volume Control Clk 44 5 14 D C 43 IIC Bus Multplexer U13203 Data

12 ( Mute )

U11902 Audio Out

+5Vr

Q3 Mute SW 12 AVR AVR Q4 Switch

SysCon U13101

IIC Bus

Head Phone Mute
(from U11800-3)

Left 7 Q2 6 Right Q1

3 2 1 U11900 +12V VCC

Sel Left Out #1

Sel Right Out #1 +12Vr Mute Head Phone Jack

Figure 15-4, Audio Output/AVR/Headphone Circuits (Repeated) The output IC's have internal protection which also limits total output current to between 3.5 to 5 amps, protecting the IC against accidental shorts in the output. A thermal protection circuit shuts the IC down if internal temperatures exceed 150C degrees. Also, internally there is a supply overvoltage protection circuit which guards against power supply voltages by shutting down the output if the power supply exceeds 50 volts.