NanoBoard 3000 - Audio System
The NanoBoard 3000 provides a high-quality audio sub-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 NanoBoard 3000 provides a 3.5mm stereo audio jack for connection of an external audio device, labeled 'LINE IN' on the board.
Audio input jack.
The signals derived from this input – LIN_R and LIN_L – are wired through to the the peripheral board connector and the on-board CS4270 Audio CODEC device.
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 | – | output from an audio resource on a peripheral board plugged-in to the motherboard. |
LineOut from the Audio CODEC | – | output from the CS4270 Audio CODEC on the motherboard |
Buzzer signal | – | this is the legacy NB1-style mono buzzer signal from the User FPGA. |
Audio Test signal | – | this signal is supplied courtesy of an additional 3.5mm audio jack (labeled
|
Note: The Buzzer signal is input to only the right channel of the motherboard's audio power amplifier. By using only the right channel, the digital noise generated by the FPGA in a sigma-delta configuration is reduced, as there is one less speaker emitting noise. When using the NanoBoard's audio system with just this signal, output is relayed to only the right-channel speaker.
Two test points are provided to monitor the output of the mixer, one for each channel – labeled 'AUL' and 'AUR' respectively.
Test points for the analog mixer.
Audio Amplification
At the heart of the audio system is a stereo audio power amplifier – a PT2300 device, from Princeton Technology. This device incorporates stereo bridged audio power amplifiers capable of producing 2W output power (into a 4Ω load), with DC volume control. It is powered by the motherboard's 5V power supply.
Stereo audio power amplifier.
Input for the device is the stereo audio signal sourced from the analog mixer, discussed in the previous section.
The PT2300 also provides a shutdown feature. This has been disabled for its operation on the NanoBoard 3000, with the control signal (SHDN) 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 (from pins -OUT1 and +OUT1) and Right (from pins -OUT2 and +OUT2).
If headphones are connected to the motherboard the PT2300's SE/BTL input is driven High, by the headphone sense signal, 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 PT2300 implemented prior to the output amplifier stage. The device has a 32-step volume controller, the output of which is determined by the voltage applied to the device's DC Vol pin. External control of this voltage (and therefore volume) is made possible through a dual gang (2x5K) potentiometer. Dual gang has been used to enhance mechanical stability on the board.
Turning the DC volume control knob (VR1) fully counter-clockwise essentially mutes the output. As the control is turned clockwise, the volume will ascend through the remaining 31 possible steps (each step adding 1.5dB) until full volume is reached when the knob is turned fully clockwise.
DC volume control.
Left (pin VO1) and Right (pin VO2) line output signals are also provided. These are the outputs of the input operational amplifiers of the PT2300 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 output signals from the audio amplifier are made available at a 3.5mm stereo audio jack, labeled 'LINE OUT' on the board. This allows you to connect, for example, your own powered speakers (the line out signals are the outputs of the PT2300's input operational amplifiers, which can drive loads greater than 1kΩ).
Audio output jack.
The negative signals of the stereo differential outputs from the audio amplifier are wired to an additional 3.5mm jack (labeled 'HEADPHONES'), used for connecting headphones to the motherboard.
Headphones jack.
The differential stereo outputs from the audio amplifier are wired to respective 2-pin KK-type connectors on the solder side of the board.
Connector sockets for the external speakers.
The NanoBoard 3000 comes with a separate stereo speakerboard. This board features 4Ω speakers and is attached to the motherboard by way of these two 2-pin connectors.
External speaker board.
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 counter-clockwise, to effectively mute the sound from the NanoBoard 3000's stereo speakers.
Location on Board
The following audio jack connectors are located along the bottom edge on the solder side of the board. Looking from the front, they can be found next to the PS/2 keyboard and mouse ports:
'HEADPHONES'jack – designatedJ6and colored Blue.
'LINE IN'jack – designatedJ4and colored Black.
'LINE OUT'jack – designatedJ5and colored Green.
'AMP TEST'jack – designatedJ7and colored Black.
Audio jack connectors (as seen from the front of the board).
The analog mixer circuitry is located on the solder side of the board, on the right-hand side.
Analog mixer circuitry.
The test points made available for the Left (labeled 'AUL' and designated TP9) and Right (labeled 'AYR' and designated TP10) stages of the analog mixer are located on the component side of the board, along with the test points for the motherboard's power supplies – to the left of the user prototyping area.
The PT2300 device (designated U26) is located on the component side of the board, to the left of the 'PLATFORM STATUS' LEDs.
The DC volume control (designated VR1) is located along the bottom edge on the solder side of the board. Looking from the front, it can be found next to the audio jacks.
DC volume knob (as seen from the
front of the board).
The 2-pin sockets for connection of left (designated J34) and right (designated J35) external speakers are located on the solder side of the board. Looking from the rear, they can be found in the top-right and top-left corners respectively.
Speaker sockets (as seen from the rear of the board).
Schematic Reference
The audio circuitry can be found on the following sheets of the motherboard schematics:
- Sheet 74 (
AUDIO_AMP_NB2C.SchDoc, entitled Audio Sub-System)
- Sheet 75 (
AUDIO_AMP_PT2300.SchDoc, entitled Audio Amp & Mixer PT2300)
- Sheet 76 (
CON_AUDIO_AC99_NOMIC.SchDoc, entitled Audio AC99 Interface)
- Sheet 77 (
CON_EXT_SPK.SchDoc, entitled External Speaker Connectors).
Further Device Information
For more information on the PT2300 device, refer to the datasheet (PT2300.pdf) available at www.princeton.com.tw.