Step 2: NAS audio input source
In the second step of the OpenNAS wizard, the user can select the audio input source interface. For each audio input source, a set of parameters related to the input interface can be set by the user. Some knowledge related to audio could be required to set the parameters according to the user's needs.
NAS offers three different audio input interfaces and a combination between two of them. Here we show a list with the NAS audio input interface options:
The AC'97 codec is mounted on a Virtex5-based board which was used at the beginning to deploy the first NAS implementations. This board offers two input interfaces: a stereo line-in connector and a stereo microphone connector. Since the audio codec is the same in both cases, there is only one spike signal processing block which converts the audio signal information from digital audio to spikes.
- Line-in: to select the stereo line-in connector.
- Mic-in: to select the stereo microphone connector. Then, you can use two microphones as ears.
- Spikes Gen bits: the number of bits of the Synthetic Spike Generator block output signal.
- Spikes Gen Clock Div: the clock divider to generate a new clock which governs the output spikes from the Synthetic Spike Generator.
- KspikesGen (GSpk./S): the maximum number of spikes (in gigaspikes per second) that can be generated by the Spike Generator module. It is calculated automatically by using the above parameters.
- Max Spike Rate: (KSpk./s): the maximum output spike rate (in thousands of spikes per second) of the AC97 VHDL module after converts audio samples to spikes. It is calculated automatically by using the above parameters.
Inter-IC Sound (I2S) is a serial bus interface standard used for connecting digital audio devices. The NAS supports this bus interface, allowing the user to design its own audio input board by using an ADC that implements the I2S bus.
- Spikes Gen bits: the number of bits of the Synthetic Spike Generator block output signal.
- Spikes Gen Clock Div: the clock divider to generate a new clock which governs the output spikes from the Synthetic Spike Generator.
- KspikesGen (GSpk./S): the maximum number of spikes (in gigaspikes per second) that can be generated by the Spike Generator module. It is calculated automatically by using the above parameters.
- Max Spike Rate: (KSpk./s): the maximum output spike rate (in thousands of spikes per second) of the I2S protocol VHDL module after converts audio samples to spikes. It is calculated automatically by using the above parameters.
Pulse Density Modulation (PDM) MEMS microphones.
- System Clock (MHz): the FPGA-based board clock frequency. This parameter is set in the Step 1, and in this step is fixed.
- PDM Clock Div: main clock divisor for PDM clock generation (which should be around 3MHz).
- PDM Clock (MHz): the PDM clock generated by the system. It is calculated automatically by using the above parameters.
- Cutt-off Freq. (Hz): cut-off frequency of the SHPF filter (Spiking High-Pass Filter) used in the PDM input preprocessing step.
- Cut-off Freq. (Hz): anti-aliasing SLPF cut-off frequency. This filter takes the SHPF output as input.
- Gain (dB): the gain of the anti-aliasing SLPF.
This option contains all the parameters listed before in 2 and 3.
In our case, we designed an audio interface board, which mounts a pair of PDM microphones and an audio ADC with I2S interface along with a stereo line-in connector. The user can select the audio input interface in real-time between either the audio ADC or the PDM microphones by mean of a jumper placed on the board. This option is an advantage, due to the user does not need to work with two different versions of the NAS to have two differents audion input interface. This way, the user spends less time.
