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astra_conv:conversion:audio:audio [2014/03/17 22:44]
richard
astra_conv:conversion:audio:audio [2014/08/22 14:09] (current)
richard
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 The audio equipment and wiring is comprised within the dashed lilac rectangular outline in the bottom center of the diagram. Radio, bluetooth receiver and DC/DC converter within a light green rectangle "​central console - upper" within the "​audio"​ rectangle. The audio equipment and wiring is comprised within the dashed lilac rectangular outline in the bottom center of the diagram. Radio, bluetooth receiver and DC/DC converter within a light green rectangle "​central console - upper" within the "​audio"​ rectangle.
 +
 +As you see, the layout is an old-fashioned approach without digital signal processing and with passive cross-overs. Being dependent on components and acoustic conditions that are far from ideal, a "​digital + active"​ approach would very much ease compensation of all the related shortcomings,​ i.e. mainly irregularities in frequency response and group delay. The "​analog + passive"​ approach here clearly comes to its limits. ​
  
  
 === radio === === radio ===
  
-The radio is a simple Blaupunkt "Essen CD31". I got it for free, and after replacing the circuit board in the operating panel and cleaning the CD player'​s lens it seems to work fine. It's main advantages are that it already ​has an auxiliary input, and it's implicit theft protection by having zero commercial value (actually, this was indeed my main reason not to look for a more uptodate model).+The radio is a simple Blaupunkt "Essen CD31". I got it for free, and after replacing the circuit board in the operating panel and cleaning the CD player ​laser's lens it seems to work fine. It's main advantages are that it has an auxiliary input, and it's implicit theft protection by having zero commercial value (actually, this was indeed my main reason not to look for a more uptodate model).
  
  
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 === active subwoofer === === active subwoofer ===
  
-This is a Pioneer TS-WX77A, a particularly flat horn speaker with 2 16cm chassis and internal 200W amplifier. ​+This is a Pioneer TS-WX77A, a particularly flat horn speaker with 2 16cm chassis and internal 200W amplifier. It allows adjustment of gain, upper edge frequency and has a phase reversal switch.
  
  
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 The thin doors of older cars require especially flat speaker chassis. I have chosen the AudioSystem "R165 flat" two-way speaker set, consisting for each side of a flat 165mm chassis, a dome tweeter and a two-way crossover. The thin doors of older cars require especially flat speaker chassis. I have chosen the AudioSystem "R165 flat" two-way speaker set, consisting for each side of a flat 165mm chassis, a dome tweeter and a two-way crossover.
-I was not convinced that the 165mm speaker would perform well at higher frequencies,​ especially what concerns its omnidirectional behaviour. So I daringly extended the front system to three ways (not counting the subwoofer), by adding midrange dome speakers (u-Dimension JR M3) and additional three-way cheapo crossovers (SinusLive CR-345). The 165 mm chassis is now only used as bass speaker.+I was not convinced that the 165mm speaker would perform well at higher frequencies,​ especially what concerns its omnidirectional behaviour. So I daringly extended the front system to three ways (not counting the subwoofer), by adding midrange dome speakers (in a first attempt) and additional three-way cheapo crossovers (SinusLive CR-345, with 700Hz and 4500Hz crossover frequencies). The 165 mm chassis is now only used as bass speaker.
  
 The following graph (source: car&​hifi) shows the frequency response of the two-way speaker set (including two-way crossovers) at two different angles. The graphs indicate that the 165mm chassis becomes highly directional at frequencies >1kHz (as is to be expected for a large cone speaker). It however also shows that this effect applies to the smaller tweeter too, at correspondingly higher frequencies. The following graph (source: car&​hifi) shows the frequency response of the two-way speaker set (including two-way crossovers) at two different angles. The graphs indicate that the 165mm chassis becomes highly directional at frequencies >1kHz (as is to be expected for a large cone speaker). It however also shows that this effect applies to the smaller tweeter too, at correspondingly higher frequencies.
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-Below frequency response and impedance of the midrange speakers (source: u-Dimension). To avoid disappointment,​ angular behaviour is not covered here.+Below frequency response and impedance of the first set of midrange speakers ​that I installed ​(source: u-Dimension). To avoid disappointment,​ angular behaviour is not covered here.
  
 {{:​astra_conv:​conversion:​audio:​mitteltoener_jrm3_spec.jpg?​600}} {{:​astra_conv:​conversion:​audio:​mitteltoener_jrm3_spec.jpg?​600}}
 +
 +Meanwhile, I have serious doubts that the shown frequency response has anything to do with the actual behaviour of these speakers.
 +
 +I later replaced them by "​conventional"​ cone midrange chassis (Gladen Zero Pro 80). Frequency response including 2nd and 3rd harmonic distortion (source: hifitest.de):​
 +
 +
 +{{:​astra_conv:​conversion:​audio:​Gladen_test_results_hifitest_c.jpg?​600}}
 +
 +Fortunately,​ with the given crossover frequencies of 700Hz and 4500Hz, they are operated in their "sweet spot", both what concerns harmonic distortion and frequency response including angular behaviour.
  
 The AudioSystem crossovers, that have very generous adjustment possibilities for the tweeter branch, remained in the dashboard and connect to the tweeters, that are mounted on top of the dashboard. Their lower frequency output, which is now set to transparent "​straight through"​ mode, is connected to the inputs of the additional three-way crossovers mounted in the doors. The AudioSystem crossovers, that have very generous adjustment possibilities for the tweeter branch, remained in the dashboard and connect to the tweeters, that are mounted on top of the dashboard. Their lower frequency output, which is now set to transparent "​straight through"​ mode, is connected to the inputs of the additional three-way crossovers mounted in the doors.
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 In both crossovers, I have removed those components that are not used, i.e. the low frequency branch for the dashboard crossover, and the tweeter branch for the door crossover. In both crossovers, I have removed those components that are not used, i.e. the low frequency branch for the dashboard crossover, and the tweeter branch for the door crossover.
  
-The mid-range ​speakers turned out to play much too loud, so I had to match their temper by a resistive ​voltage ​dividerSurprisingly,​ an attenuation near 15dB was required (Please note that the drawing on the photo is outdated)The resistive character of the voltage divider ​will also support the operation of the crossover, since the impedance of the speakers alone is strongly frequency dependent. In particular, the impedance rise at higher frequencies,​ due to the speaker coil's inductivity,​ may prevent a clean cut-off at a given upper edge frequency.+In each of the front doors' stacking tray, I provided a small black box with wire connections to bass and mid-range ​chassis'​. It allows ​to add e.g. reisistive ​voltage ​dividers to reduce the volume or to reverse the phase of the mid-range chassis. Please note that the drawing on the photo is outdated. ​ 
 + 
 +Resistors in parallel to the speakers ​will also support the operation of the crossover, since the impedance of the speakers alone is strongly frequency dependent. In particular, the impedance rise at higher frequencies,​ due to the speaker coil's inductivity,​ may prevent a clean cut-off at a given upper edge frequency.
  
 {{:​astra_conv:​conversion:​audio:​p1100864.jpg?​600|}} {{:​astra_conv:​conversion:​audio:​p1100864.jpg?​600|}}
  
-The bass chassis and the tweeters ​have no resistive dampening yet, which may leave further room for improvement.+Currently, ​the chassis ​have no resistive dampening yet, which may leave further room for improvement.
  
  
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 {{:​astra_conv:​conversion:​audio:​p1100380.jpg?​600|}} {{:​astra_conv:​conversion:​audio:​p1100388.jpg?​600|}} {{:​astra_conv:​conversion:​audio:​p1100380.jpg?​600|}} {{:​astra_conv:​conversion:​audio:​p1100388.jpg?​600|}}
  
-The mid range speakers ​(which have a closed camber) ​were mounted on wooden spacer rings, but due to their depth also required cutouts in the door panels. The ugly blain in the cover below the cutout even was intentional,​ though it came out misshapen: The new bass chassis have a slightly higher diameter, so I had to mistreat the plastic cover with the hot air blower to free some space for them.+The mid range speakers were mounted on wooden spacer rings, but due to their depth also required cutouts in the door panels. ​
  
-{{:​astra_conv:​conversion:​audio:​p1100381.jpg?600|}} {{:​astra_conv:​conversion:​audio:​p1100383.jpg?​600|}}+For the cone speakers that I installed later, I used wedge-shaped adapter rings to slightly tilt the chassis back - and upwards.
  
-Re-assembled front door with bass chassis and midrange speaker. ​+{{:​astra_conv:​conversion:​audio:​p1120163.jpg?​600|}}  
 + 
 +{{:​astra_conv:​conversion:​audio:​p1120165.jpg?​600|}}  
 + 
 +{{:​astra_conv:​conversion:​audio:​p1100383.jpg?​600|}} 
 + 
 +The ugly blain in the cover below the cutout even was intentional,​ though it came out misshapen: The new bass chassis have a slightly higher diameter, so I had to mistreat the plastic cover with the hot air blower to free some space for them. 
 + 
 +Re-assembled front door with bass chassis and midrange speaker ​- first attempt ...
  
 {{:​astra_conv:​conversion:​audio:​p1100959.jpg?​600|}} {{:​astra_conv:​conversion:​audio:​p1100959.jpg?​600|}}
 +
 +and the second, final version:
 +
 +{{:​astra_conv:​conversion:​audio:​p1120183.jpg?​800|}}
 +
  
 \\ \\
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 I have already spent some time tuning the sound via the numerous setting possibilities of the dashboard crossovers (tweeter frequency response and volume), high-pass and subwoofer amplifiers (edge frequency and gain), and by trying different resistor combinations for the mid-range speakers'​ voltage dividers. For the mid-range speakers'​ polarity, I decided (rather a guess) not to reverse it. I have already spent some time tuning the sound via the numerous setting possibilities of the dashboard crossovers (tweeter frequency response and volume), high-pass and subwoofer amplifiers (edge frequency and gain), and by trying different resistor combinations for the mid-range speakers'​ voltage dividers. For the mid-range speakers'​ polarity, I decided (rather a guess) not to reverse it.
  
-Meanwhile, the radio sounds somewhat ok  - mission accomplished!+After a whileI was inclined to believe that the radio sounds somewhat ok  - mission accomplished! 
 + 
 +Later however I had to realize that the sound was actually not so good, particulary in relation to the effort I had invested. The initial, positive impression probably came from always listening to the same few "test tracks",​ and from underestimating how quickly and how much you can get used to a speaker system'​s linear distortions. It also seems virtually impossible to derive the right "​corrective action"​ to a perceived flaw in the frequency response without the help of an objective measurement. 
 + 
 +Another surprise was to see (or hear), by how much the sound perception changes under the influence of rolling noises. It seems that especially when listening at low volume, the noise floor will first compete with the "​dips"​ in the frequency response, masking out the corresponding parts of the spectrum and thus making the "​peaks"​ much more prominent.  
 + 
 +Rolling noises, by the way, have their highest portions at low frequencies,​ asking for a deliberate enhancement of the bass and sub-bass bands to keep them audible at low volume. 
 + 
 +After quite some fruitless efforts to get the system to sound well, I decided to objectively record the frequency response. 
 + 
 +##############################################################​ 
 + 
 +Please note: The following measurements were all done before I replaced the mid-range speakers. With the second version that I used, I was able to get a proper sound out of the system, by just adapting the levels of the different chassis. I so far have not repeated the measurements with the new setup. 
 + 
 +##############################################################​ 
 + 
 + 
 +This eigthies'​ electret condensor microphone is placed beneath the passenger seat's headrest.  
 + 
 +{{ :​astra_conv:​conversion:​audio:​p1110895_c.jpg?​500 |}} 
 + 
 + 
 +A sinusodial test tone generated by an android tone generator is fed to the radio via bluetooth streaming. 
 +The DJ mixer acts as microphone preamplifier. The amplified signal is "​quantified"​ by a volt meter.  
 + 
 + 
 +{{ :​astra_conv:​conversion:​audio:​p1110898.jpg?​800 |}} 
 + 
 + 
 +For each of a row of frequencies,​ a fully automated test support chimp sets the tone generator to a certain freqency, restarts it in case it has another "​buffer overflow"​ and enters the AC voltage reading into a spreadsheet.  
 + 
 +The spreadsheet transforms the readings into a logarithmic scale and generates the chart below.  
 + 
 +It shows the results from the Astra'​s car stereo, and two reference curves. Their "​polygonal"​ appearance is due to the low number of samples taken. The car stereo measurement has not been corrected by subtracting a reference curve. So, a "​perfect"​ speaker system should not yield a flat curve, but rather a curve resembling reference 1. 
 + 
 +Generally, it was a bit disappointing how much irregularities an acoustic frequency response seems to have, also compared with curves one usually gets to see. Obviously, a higher number of tests points would have been useful. 
 + 
 + 
 +pink curve (reference 1): frequency response of the microphone, as from it's datasheet. 
 + 
 +blue curve (reference 2): frequency response of my "​living room stereo"​ speakers (Marantz LD200), old, but still sounding very well. Note that the bass chassis once have been replaced (by a professionist) and the bass reflex system has apparently not been matched to the new speakers'​ properties. 
 + 
 +For this measurement,​ the microphone was placed approx. 1m in front of the speaker, in a moderately dampened room. Apart from the bass resonance peak and dip, quite some irregularities are visible. 
 + 
 +dark red curve: frequency response of the Astra car stereo (somewhen during the "​optimization"​ process). In the car and at higher distance from the speakers, the sharp irregularities are much more prominent that in the "​living room" case. This is is probably due to reflections within the car. 
 + 
 +The most ugly peaks are at 50, 100 and 400Hz. The 400Hz peak might be due to a cavity resonance from inside the front doors. The 50Hz and 100Hz peaks are apparently caused by cavity resonances of the car's interior itself. The 50Hz peak was additionally enhanced by a rattling noise provoked by that resonance. Both peaks are probably not easy to tackle, except by electrically suppressing these frequencies.  
 + 
 +The range between 700Hz and 4kHz is just too low by some 5..6dB. This can be easily corrected by turning up the volume (i.e. modifying the resistive attenuator) of the mid- range speakers. Also raising the high frequency end is only a matter of some settings of the crossovers. 
 + 
 + 
 + 
 + 
 +{{:​astra_conv:​conversion:​audio:​freq_response07.jpg?​900|}} 
 + 
 + 
 +Apart from that - it seems that here I am at a point where I can either pretend that I am happy with the sound, or I have to bear the effort of integrating a digital sound processor. Starting with this, probably also the passive cross-overs would have to give way to a few more amplifiers... 
 + 
 + 
 +##############################################################​ 
 + 
 +Please note: I had drawn above conclusion before I replaced the crappy midrange speakers. With the second set of speakers, the sound is now much better, without any large investments like sound processor or active crossovers. 
 + 
 +Before the speaker swap, I even had tried to integrate a sound processor. I returned it shortly after, since the sound got "​artificial"​ rather than "​good"​. ​
  
-A more serious approach will require objective measurement of the frequency response - and hopefully it can then also be successfully levelled out.+##############################################################​
  
  
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astra_conv/conversion/audio/audio.1395096247.txt · Last modified: 2014/03/17 22:44 by richard