Audio VU meter (AC microVoltmeter) with Extra wide Dynamic Range 69 dB.
O’K, after having some fun with stereo version of the VU meter I described in my previous blog-post, now it’s time to do a serious stuff. Studio grade VU meter !!! 24 steps, equally spaced every 3 dB, covering Extra wide Dynamic Range from -63 up to +6 dB. Single (mono) channel this time, no messing around, absolute precision at the stake. Plus, it keeps absolutely Top-Flat linear frequency response from 40 Hz up to 20 kHz(*).
I’m not going into details of RGB LEDs Display, which has no modification since “Tears of Rainbow” project, only plates installed in one line, form a single GIGANTIC bar-graph. There are some minor changes in mixing colors data tables, but they intuitively understandable. The most important feature in this project is autoscaling. As you, probably know, Arduino has 10 bits ADC. Only it can’t process negative half-wave, and for this reason it has only 9 bits available for AC measurements. According to DSP theory, maximum dynamic range is:
DR = 1.77 + 6.02 x B = 1.77 + 6.02 x 9 = 55.95 dB.
As input audio waveform represents anything but perfect peak-to-peak 5V sine-wave, real dynamic range would be lower. How much? In first, there is a hardware limits. OPA (NE5532), which is:
- very low noise !!!
- high output-drive capability;
- high unity-gain and maximum-output-swing bandwidths;
- low distortion;
- high slew rate;
- input-protection diodes, and output short-circuit protection
but, unfortunately, isn’t rail-to-rail type. Test results show, that compression become noticeable (~1 dB) when not scaled magnitude approaches level about 50 dB. That is in good agreement with observed on oscilloscope not distorted deviation peak-to-peak 2.5 V. Or only half of full range of 5V. And as theory says, half is one bit less, and real DR = 1.77 + 6.02 x 8 = 49.93 (~50 dB). In second, audio data is processed on “block” structure basis. It means, having average of the block 50 dB, doesn’t mean that there was no spikes in the sampling pull, that obviously would be clipped and introduce error in the measurements results. This phenomenon is defined as Crest Factor. Different sources estimate crest factor of musical content between 10 – 20 dB. So, taking direct approach, Arduino with OPA mentioned above as front-end could accurately cover only: 50 – 20 = 30 dB. To get wider dynamic range, I have to scale input amplifier gain, and this is exactly what I did, building amplifier in two stages and selecting one cascade (by-passing second one) or two cascades using internal ADC multiplexer. As there is no switching IC in analog signal path involved, gain is defined with high stability, could be one time precisely measured – calibrated via coefficient stored in EEPROM (nice feature to add).
On the right side there are electrical drawings of “slightly” modified kit, where stereo amplifier was converted into 2 stage mono version. First stage, with gain about G1 = 1 + 10 k / 1 k = 11 is necessary to “bump-up” line-level signal, to create DC bias required for correct operation of the ADC, and also served as buffer to lower signal source impedance, as it seen by ADC input. I set a gain of the second stage amplifier at 40 dB: 20 x Log_10 ( G2 ), where G2 = 1 + 100 k / 1 k = 101.
IMHO, setting gain limit for only 30 db per stage as it follows from paragraph above, is overkill, and would be justified for “real-time” radio broadcasting or audio processing for storage media, when high fidelity of audio program must be preserved. For visual display “clipping” of bursts in signal is not noticeable at all due high refresh rate of display, 78 Hz. Human just can’t see, if LED lights-up with such speed. For steady AC amplitude measurements (micro Voltmeter mode) this is not a problem at all, and headroom as small as 3 dB would be sufficient, leaving wide 47 dB per stage.
Software
There are two thresholds are defined in program, where switching between one or two stage amplification is happening:
if ( magn_new <= 44 ) sensitv = 1;
if ( magn_new >= 47 ) sensitv = 0;
44 and 47, with hysteresis 3 dB. First line defines switching to high sensitive mode (overall gain 1100), and second line, does exactly opposite. Look at the chart, hope it would save me a million words -);
Couple words on using this device as precise AC micro-voltmeter. Having 1100 overall amplification as add-up to already quite sensitive Arduino ADC, driving overall sensitivity to enormously 5 / ( 1024 x 1100 ) = 4.439 uV Special care should be taken on grounding, shielding of amplifier PCB, probably, EMI suppressor ferrite chokes wouldn’t be an excess in power line and signal path. In my project, w/o any modification to original kit’s board (except couple jumper wires to cascade two stage amplifier) of course, I was not expecting to get to such high sensitivity level. Moreover, in project arduino is driving LED display, “ADC noise reduction mode” is off, plus ADC is working on double speed – preselector set to 250 kHz!!! And this is why constant 14 was subtracted in software from magn_new, just before it goes for BarGraph “mapping” procedure:
magn_new -= 14;
Basically 14 is a noise flour of my analog front-end. Approximately 51 micro volts AC is turning on first LED bar. Look at the table, which reflect my current hardware set-up.
* Other things to keep in mind, there is a “gap” 78 Hz wide in frequency range at 10 kHz, It introduces a small error, about 78 / 20.000 = 0.39% in white noise measurements result. For musical content, which has really low power density level at 10 kHz, magnitude of error would be much lower, probably, less than 0.05 %.
Running FFT in code creates great opportunity to reject any interference in the audio band. For example, if there is a noticeable hum from electrical grid lines in the content, issue easily could be fixed NOT including bin[1] in final sum of magnitude calculation. Though to make it works more efficient, some adjustment in sampling period would be necessary, setting bin[1] frequency precisely at 50/60 Hz.
One more advantage of having FFT based filtering (primary mission is HPF, look in stereo VU meter, how long kernel of the FIR filter has to be otherwise), is great opportunity to create “weighting” A, B, C or D curve for audio noise measurements. (:TO DO).
Link to Download Arduino sketch: Audio_VU_Meter_Mono_69dB
