NB2DSK01 - Stereo Audio
The NB2DSK01 provides a powerful stereo audio system, complete with analog mixer, power amplifier and various sound output methods. The following sections take a closer look at the various stages involved in the implementation of this system.
Sound Input
The NB2DSK01 provides 3.5mm stereo audio jacks for connection of an external audio device ('LINE IN') and a microphone ('MICROPHONE').
Figure 1. Audio input jacks and mixer channel
test points.
The signals derived from these inputs – LineInR, LineInL and MicIn – are wired through to each of the peripheral board connectors. The connectors are generic with respect to their pin assignments. In this way, a peripheral board containing an audio-based resource can be placed in any of the three connector positions, and the audio input signals are made available.
Prior to being sent to the peripheral board connectors, the MicIn signal is fed through a MAX9812L low-power microphone amplifier(from Maxim). Powered by the NB2DSK01's 3.3V supply and permanently enabled, the amplifier delivers a fixed gain of 20dB.
An analog mixer is used to mix the following audio sources together (one stage each for Left and Right channels):
LineOut from Host | – | this is the output from a Delta-Sigma DAC, driven by the host (NanoTalk Controller) FPGA. |
LineOut from peripheral board A | – | output from an audio resource on a peripheral board plugged-in to the socket corresponding to |
LineOut from peripheral board B | – | output from an audio resource on a peripheral board plugged-in to the socket corresponding to |
LineOut from peripheral board C | – | output from an audio resource on a peripheral board plugged-in to the socket corresponding to |
Buzzer signal | – | this is the legacy NB1-style mono buzzer signal from the daughter board FPGA. |
Audio Test signal | – | this signal is supplied courtesy of an additional 3.5mm audio jack. Used as a test input to the amplifier, it essentially gives you a direct Line In from an audio source external to the NB2DSK01. |
Two test points are provided to monitor the output of the mixer, one for each channel.
Audio Amplification
At the heart of the audio system is a stereo audio power amplifier – an LM4849MH Boomer® device, from National Semiconductor. This device incorporates stereo bridged audio power amplifiers capable of producing 2W output power, with DC volume control. It is powered by the NB2DSK01's 5V power supply.
Figure 2. Stereo audio power amplifier and related circuitry.
The device supports two stereo input signals, with an internal multiplexer to choose between them. For the NB2DSK01, the multiplexer control signal has been tied to ground, ensuring that only stereo input 1 is used. This input is sourced from the analog mixer discussed in the previous section.
The LM4849 also provides shutdown and mute features. Both of these are disabled for its operation on the NB2DSK01, with the relevant control signals tied to ground.
The stereo amplifier in the output stage of the device is formed by wiring two pairs of operational amplifiers into a 'bridged' configuration. The result is stereo differential output – Left Out +/- and Right Out +/-.
If headphones are connected to the NB2DSK01 the LM4849's headphone sense input is driven High, causing the bridged output to effectively be muted – turning off the amplifiers driving the positive signals of the Left and Right channel differential outputs. In this case, the device is placed into single-ended output mode.
Both bridged and single-ended outputs can be adjusted in terms of volume – the DC volume control stage of the LM4849 implemented prior to the output amplifier stage. Volume control is made possible through a dual gang (2x5K) potentiometer. Dual gang has been used to enhance mechanical stability on the board.
Figure 3. DC volume control.
Left and Right line output signals are also provided. These are the outputs of the input operational amplifiers of the device, which in turn provide the inputs to the device's internal volume control stage. As such, these line output signals are not volume-adjustable.
Sound Output
The line outputs (LLOUT, RLOUT) are made available at a 3.5mm stereo audio jack. This allows you to connect, for example, your own powered speakers (the line out signals are the outputs of the LM4849's input operational amplifiers, which can drive loads greater than 1kΩ).
The negative signals of the stereo differential outputs (LOUT-, ROUT-) are wired to an additional 3.5mm jack, used for connecting headphones to the NB2DSK01 motherboard.
Figure 4. Audio output jacks.
Desktop Stereo Speaker Assembly
The differential stereo outputs (LOUT-, LOUT+ / ROUT-, ROUT+) and the DC volume control signal are wired to a 16-pin connector, on the solder side of the motherboard.
Figure 5. Connector socket for Desktop Stereo
Speaker Assembly.
The Desktop NanoBoard NB2DSK01 comes with a separate stereo speakerboard – the Desktop Stereo Speaker Assembly NB2DSK-SPK01. This board features 4Ω speakers and comes already affixed to the motherboard by way of this 16-pin speaker board connector.
In addition to the audio-based signals, the connector provides two SPI buses (the associated signals of which are: LED_DIN1, LED_SCK1, LED_CS1 and LED_DIN2, LED_SCK2, LED_CS2) to the speaker assembly, used to control RGB LED drivers.
A 1-Wire bus signal is also provided – AUD_1WIRE – which is routed from the NanoTalk Controller to pin 14 of the connector. This provides a path with which to interrogate the 1-Wire ID device located on the speaker board.
This connector also provides 3.3V, 5V, audio ground and power ground signals.
Figure 6. Desktop Stereo Speaker Assembly NB2DSK-SPK01.
Should you wish to connect your own external speakers via the 'LINE OUT' jack, you will need to turn the volume control pot all the way down, to effectively mute the sound from the NB2DSK-SPK01's stereo speakers.
Location on Board
The following audio jack connectors are located on the solder side of the board, between the parallel port connector and the RS-232 port:
'MICROPHONE'jack – designatedJ11and colored Pink.
'LINE IN'jack – designatedJ10and colored Black.
'LINE OUT'jack – designatedJ12and colored Green.
'HEADPHONES'jack – designatedJ13and colored Blue.
The LM4849 device (designated U17) and analog mixer circuitry are located on the solder side of the board, directly below these four audio jacks.
The MAX9812L device (designated U18) is located on the solder side of the board, below the 'MICROPHONE' jack.
Figure 7. Low-power microphone
amplifier (U18).
The 'AUDIO TEST' jack (designated J14 and colored Black) is also on the solder side of the board, directly above the 'SYSTEM JTAG' header.
Figure 8. Audio test input.
The 16-pin socket for connection of the Desktop Stereo Speaker Assembly (designated J15 and labeled 'TO NB2DSK-SPK01') is located on the solder side of the board.
The DC volume control (designated VR2) is also located on the solder side of the board, next to the SD Card reader.
The test points made available for the Left (TP13) and Right (TP14) stages of the analog mixer are located on the component side of the board, below the labeling for the 'MICROPHONE' audio jack.
Schematic Reference
The audio circuitry can be found on the following sheets of the motherboard schematics:
- Sheet 65 (
AUDIO_AMP_NB2A.SchDoc, entitled AUDIO SUB-SYSTEM)
- Sheet 66 (
AUDIO_AMP_LM4849.SchDoc, entitled AUDIO AMP & MIXER LM4849)
- Sheet 67 (
AUDIO_AMP_MAX9812L.SchDoc, entitled Microphone Pre-Amplifier)
- Sheet 68 (
CON_AUDIO_AC99.SchDoc, entitled AUDIO AC99 Interface)
- Sheet 69 (
NB2_EXTEND_A.SchDoc, entitled NB2-Extender A Interface)
- Sheet 70 (
CON_NB2_EXTEND_A16.SchDoc, entitled NB2-Extender A 16-Pin Header).
Further Device Information
For more information on the LM4849 device, refer to the datasheet (LM4849.pdf) available at www.national.com.
For more information on the MAX9812L device, refer to the datasheet (MAX9812-MAX9813L.pdf) available at www.maxim-ic.com.