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In the March and April 2010 issues, we described the design and construction of our new Digital Audio Signal Generator. The final article this month has the driving instructions. By NICHOLAS VINEN A High-Quality Digital Audio Signal Generator; Pt.3 D RIVING THE Digital Audio Signal Generator is straightforward. In operation, it delivers an output signal (analog and/or digital, depending on the configuration) as soon as it is switched on and the LCD initially shows the current signal generation mode. There are five such read-outs, one for each mode: locked, independent, mixed, pulsed and sweep. In each case, the Up and Down buttons change the current mode and the display adjusts to show the corresponding reading. Pressing the Select button (in the centre of the main cluster) switches the LCD from the signal generation mode read-out to the setting read-outs. When this is done, signal generation continues according 74  Silicon Chip to the last mode selected. Changing the setting read-out (done using the Up and Down buttons) has no affect on the current signal generation mode. There are seven different setting read-outs. Pressing the Select button again returns the unit to the signal generation mode read-out. In other words, the Select button toggles between the current signal generation mode read-out and the setting read-out. The active generation mode is always the mode which was last selected. Left & right buttons On most read-outs, the left and right buttons allow you to move a cursor across the display. The only excep- tion is the Status read-out where these two buttons have other effects (more on this later). In practice, the cursor can only move to locations which show values that can be adjusted or activated. When the cursor is visible, pressing the Up and Down buttons will modify the indicated setting rather than changing the current read-out. Pressing the Select button or moving the cursor past the first or last setting hides the cursor and the Up and Down buttons can once again be used to change the current mode and/ or read-out. This system may sound complicated but once you try it, it will quickly start to make sense. In other words, it siliconchip.com.au sounds more complicated that it really is and the process is quite intuitive once you understand the basics. Table 1: Signal Generation Mode Readouts Locked Mode Readout: top line = signal frequency (both channels) and left channel amplitude; bottom line = channel phase difference and right channel amplitude. The mute buttons There are two additional buttons on the unit – Left Mute and Right Mute. Pressing them toggles the mute status of the corresponding channel at any time. For example, if you press the Left Mute button and the left channel is currently enabled, it will be disabled and vice versa. The mute status is shown on the status read-out and also on each mode read-out. For sweep mode, if a channel is muted, an “l” (indicating the left channel) and/or an “r” (indicating the right channel) is shown at the top of the display. For the other four modes, a muted channel is shown by changing the minus sign in front of each amplitude setting to an underscore. If you press the two Mute buttons simultaneously, the left and right channel settings will be swapped. This includes frequency, amplitude and phase (when applicable), as well as the mute status. Table 1 shows what each generation mode read-out looks like by default and highlights all the settings that can be changed. There is also a description of the function for each setting. More mode information As stated in the original article, the output frequency can be set at up to half the sample rate (ie, 48kHz) in steps of 1Hz. Depending on which frequency digit you select with the cursor, pressing the Up and Down buttons will add or subtract 1Hz, 10Hz, 100Hz, 1000Hz or 10,000Hz. The amplitude is set in similar fashion and the range is from 0dB to -98dB in 1dB steps. If you increase the attenuation past -98dB, the read-out changes to “off” and the signal is muted. Note that as you get close to -98dB, the actual signal amplitudes become so small that the error increases and some values have an identical result. In fact -96dB, -97dB and -98dB generate the same amplitude due to the 16-bit precision of the scaling factor. However, for attenuation down to -60dB, the scaling is basically perfect. Beyond that, the measured values are as shown in Table 2. Note that these measurements also include scaling errors from the DAC siliconchip.com.au Independent Mode Readout: top line = left channel frequency and amplitude; bottom line = right channel frequency and amplitude. Mixed Mode Readout: top line = frequency and amplitude of wave 1; bottom line = frequency and amplitude of wave 2. Pulsed Mode Readout: top line = frequency (both channels) and pulse-on amplitude; bottom line = pulse-on time (0-9999ms), pulse-off time (0-999ms) and pulse-off amplitude. Sweep Mode Readout: top line = start frequency and amplitude (both channels); bottom line = finish frequency (both channels), sweep time (0.1 - 99.9s) and off-time (0-98s & manual). Note: values inside green boxes can be selected and varied using the front panel pushbuttons. itself, so they are only a guide as to the unit’s actual precision. Keep in mind that when adjusting the attenuation, pressing the Up button increases the attenuation and thus decreases the signal amplitude. The only other mode settings (aside from frequencies and amplitudes) are time periods. In the case of pulsed mode, they are specified in milliseconds and have a range of 0-9999 (just under 10 seconds) or 0-999 (just under one second). In sweep mode, they are specified in tenths of a second and seconds, with a range of 0-99.9 seconds and 0-99 seconds respectively. If you switch away from a mode and then back again later, the previous settings are typically retained. However, some are shared between the modes. For example independent and mixed mode share all their settings, differing only in how they output the signal (independently to each channel or mixed on both). As a result, changing a setting in one changes both. The left/right channel amplitudes are shared between all modes except pulse and sweep. Most other settings are independent. Configuration details Table 3 shows each setting read-out and describes each field. Some require more explanation, as follows: Status readout: the Status readout is very useful in sweep mode. Not only Table 2: Attenuation Accuracy Setting Measured Value -60dB -59.92dB -70dB -69.80dB -80dB -79.16dB -90dB -89.35dB does it show the frequency as the sweep occurs (it’s updated four times a second) but you can pause and restart the sweep. Pressing the Left button in this readout mode pauses or resumes the sweep, while pressing the Right button starts/restarts it. This is especially useful when you are optimising crossover networks or matching a subwoofer to other drivers. If you hear a peak in the output amplitude, you can pause the sweep and read off the frequency. There may be times when you want a manual sweep, ie, rather than having it loop repeatedly, you can trigger it manually. To do this, set the sweep off time to maximum via the sweep mode read-out – it will show “man” (manual). It will then only start when you trigger it manually from the status read-out using the Right button. Sweep setting readout: this simply allows you to choose between Exponential or Linear sweep. Exponential sweep is the default and is usually May 2010  75 Table 3: Setting Readouts Output/wave type setting read-out: top line = output type (44100Hz, 48000Hz [default], 96000Hz, Analog); bottom line = signal type (Sine, Square, Triangle, SawtoothUp, SawtoothDn). Status read-out: indicates the current generation mode and mute status. In pulsed and sweep mode, it shows the current frequency and amplitude. “LR” becomes “lr” when both channels are muted. Sweep setting read-out: first line = setting; second line = sweep type (Exponential [default] or Linear). S/PDIF setting read-out: line 1 = encoding type (Consumer [default] or AES/EBU) plus pre-emphasis setting (Normal [default] or PreEmph); line 2 = battery voltage display. Battery setting read-out: Line 1 = 3.3V regulator output voltage calibration; line 2 = low battery warning voltage setting. LCD setting read-out: line 1 = LCD backlight brightness (0-100%); line 2 = LCD contrast (0-100%). EEPROM setting read-out: line 1 = Select EEPROM bank (0-9); line 2 = Load all settings from specified bank and Save all settings to specified bank. Note: values inside green boxes can be selected and varied using the front panel pushbuttons. what you want. In this mode, the frequency doubles or halves at a fixed rate. Linear sweep simply increases the frequency by a set amount over time. As a result it spends less time at low frequencies and more time at higher frequencies. This mode could be useful if you are using a computer to capture and analyse the output, as it may make analysis simpler. Keep in mind that while the generator attempts to reach your programmed finish frequency after the specified time has elapsed, in practice this is very difficult to achieve. As a result, with long sweep times, it may be off by a few milliseconds and occasionally the final frequency may actually be a few Hz below that specified. S/PDIF setting read-out: this lets you configure the S/PDIF output format as well as view the current battery voltage. The first setting, “Consumer” or “AES/EBU”, determines the format of the Channel Status data sent with the S/PDIF stream. “Consumer” is the typical format that most CD and 76  Silicon Chip DVD players use. On some equipment, this format is limited to 20 bits of precision in the audio data, so you may get slightly higher distortion and worse amplitude control on this setting. However, it is the most widely supported. “AES/EBU” is the professional standard used by DATs, mixers and high-end sound cards. It allows the full 24-bit precision in the audio samples as well as sending more complete meta-data. In general, AES/EBU mode is better provided the equipment that’s receiving the signal can handle it. The second setting allows you to enable the pre-emphasis bit in the S/PDIF stream. This has the effect of enabling the de-emphasis hardware in the receiver (if it has any) and is mainly useful for testing. For example, you can run a 20Hz-20kHz sweep with and without this bit set and check that the higher frequencies are properly attenuated (ie, de-emphasised) when it is set. The battery voltage readout lets you keep track of the charge state. Note, however, that the reading is actually a little lower than the real battery voltage (by about 0.3V) due to the Schottky diode (D2) in series with it. This means that if you consider your cells flat when the battery reaches 4.0V (for example), you’ll actually want to switch the device off or attach the plugpack as soon as it reads near 4.3V. Battery setting readout: this lets you calibrate the battery voltage monitor – see the “Calibration” section in Pt.2 in the April 2010 issue. It also lets you set the low-battery warning voltage threshold. Once the battery has dropped below this level, the backlight will dim and flash, warning you to charge the battery or switch it off. During this time, you can continue using the generator. The recommended settings are 3.9V for alkaline cells (actually 3.6V) and 4.3V for NiMH cells (actually 4.0V). Regardless of this setting, if the voltage reading drops below 3.5V for some time, the microcontroller will go into sleep mode. The “Battery flat!” message is then displayed and all other functions cease. Backlight flashing continues although at a reduced brightness. When that happens, the current drain drops from 100mA or more to about 30mA. This will still drain the battery but not as quickly. LCD setting readout: here you can adjust the display contrast and backlight brightness. You may need to change them according to lighting conditions, viewing angle or temperature. If you manage to reduce the contrast so much that the display becomes unreadable, switching the power off and on will typically restore it to the default. The backlight brightness selection is a compromise because as you increase it, you increase the current drain at the expense of battery life. The default value of 25% is sufficient for good display visibility under most conditions and only adds about 30mA to the battery current. EEPROM setting readout: this lets you define the default settings (ie, those loaded at power on), as well as store up to nine other setting configurations. Bank 0 is loaded at switch-on, so if you save to bank 0 you are setting the defaults. To save settings, select the appropriate bank number, then move the cursor to “Save” and press the Up or Down button. The read-out will change to “Saved” and the current configuration and mode settings will be stored in that siliconchip.com.au slot. You can then switch the generator off, or continue using it. You can even change the bank number and save to another one if you wish. To load settings, the procedure is essentially the same. You select the bank number you want, move the cursor to “Load”, and press the Up or Down button. The read-out changes to “Loaded” and the current settings are replaced with those stored in EEPROM. Almost everything will be set just as it was when you saved to that bank. Note that attempting to load a bank that has nothing saved in it has no effect. The additional banks can be handy if you often repeat certain tests and they involve a specific configuration. You can store commonly used configurations in banks 1-9 and save yourself the time of having to adjust multiple settings to the same values again later. Table 4: Example Wave Types Internal DAC Sine External DAC Sine Wave types Table 4 gives an example of each wave type. Each has been sampled using both the analog outputs of the Digital Audio Signal Generator and also the SILICON CHIP High-Quality Stereo DAC (September-November 2009), the latter fed from the generator’s S/PDIF output. The high-frequency oscillations apparent in both the square and sawtooth signal outputs from both DACs are a result of their delta-sigma architecture. These types of waveforms are unnatural due to their discontinuous nature – ie, they contain vertical lines whereas natural waveforms normally do not. As a result, the digital sinewave signal interpolation is smoothing the abrupt transitions, with the filter inserting some high-frequency waves before and after each transition to cover up the discontinuity. The result is what you see here. We know that the digital circuitry must be responsible for the oscillations because they occur equally both before and after each transition. On the other hand, the external DAC has some overshoot with the square and sawtooth waves, which is presumably due to the analog filter circuitry after the DAC chip itself. It only occurs post-transition. The sinewave signal does not suffer from this issue because it is continuous. The triangle signal does have a discontinuous first derivative (at the point of each triangle) but since it has a siliconchip.com.au Square Square Triangle Triangle Sawtooth Up Sawtooth Up Sawtooth Down Sawtooth Down May 2010  77 Fig.1: the effects of aliasing start to become noticeable at 44kHz. Note the subtle variations in the signal amplitude. Fig.2: at 47kHz, aliasing effects are quite severe (the input data in this case actually has constant amplitude). Fig.3: this scope grab shows the analog output distortion residuals (0.06%) for a 1kHz sinewave & 0dB attenuation. Fig.4: the distortion residuals drop to just 0.0006% (1kHz, 0dB attenuation) when using the High-Quality Stereo DAC. continuous waveform it is not badly affected. There is a little rounding at the tips, again likely due to the digital sine signal interpolation, but it is minimal. Aliasing There is an additional issue related to the digital filtering and that is aliasing. In fact, all DACs suffer from it to some extent. When the sample rate is set to 96kHz, the highest frequency signal you can generate is 48kHz. There is a wrinkle, though – as you get very close to 48kHz, there are so few samples per signal that the signal form can no longer be properly represented. Basically, the digital data becomes ambiguous – while the frequency information can still be extracted, the amplitude of each wave is no longer clear. Aliasing starts to be become no78  Silicon Chip ticeable above 44kHz (note the subtle variations in amplitude shown in the scope shot of Fig.1) and it is quite severe at 47kHz (Fig.2). The input data for the second capture actually has a constant amplitude. However, aliasing does not increase monotonically with frequency. There is no aliasing at exactly 48kHz, for example. What this means is that you should generally avoid frequencies between 45% and 50% of the sample rate, except for exactly half (ie, 48kHz in this case). That way, aliasing will not typically be an issue. By the way, if you want to make sound effects for a science fiction film, try setting the signal type to triangle and the sample rate to 48kHz, then initiate a sweep from 12-24kHz. What you then hear is due to the aliasing causing frequency shifts in the output. Distortion There’s another issue to keep in mind when selecting the sample rate. When the output frequency is below about 10kHz, the sinewave signal distortion is actually lower at 48kHz sampling than at 96kHz sampling. So if you’re only going to generate low-tomid frequency sinewaves and need the least distortion, stick with the 48kHz sampling rate. You can still switch to 96kHz when necessary for higher frequency output signals. Finally, if you have an oscilloscope, it’s a good idea to connect the generator’s outputs to it and try out the various modes. By doing this, you will quickly get used to the interface and get a feel for how the various modes SC work. siliconchip.com.au