DIY Time Alignment.

[lms]

alpine supplies some of the honda head units

[clipto333]

Originally posted by Eva
In the analog domain you can try "all-pass filters" but each stage is limited to only 90 degrees of linear delay at the highest frequency desired, thus a lot of stages are required to get something like 0.7ms up to 20Khz (0.7ms is the average left-to-right channel error for European cars). These analog filters are best suited to delay only a (sub)woofer or a midrange within its (crossover limited) frequency range.

You can also find out about "bucket brigade" delay ICs, but they are not exactly hi-fi. They are mostly used in low cost reverb circuits in musical instruments.

The third option involves converting the signal to the digital domain, storing it to RAM and then retrieving it and converting it back to analog in real time with a fixed delay. This is the DSP approach. There are low-cost ready-made solutions with lots of useful features, like Behringer DCX2496, that may be modified for car audio usage.

BTW: Some Alpine headunits include adjustable delays, parametric EQ and crossover too, but an entry-level pro-audio DSP like DCX2496 has far more gadgets in it if you really enjoy experimenting with sound.

hi,

i agree, it would be difficult to TA in analogue domain. is a dsp difficult to build? i would like totry it even if it is. buying a dsp is too easy a solution and wont be satisfied if i dont diy.

i think, bucket brigade is quite obsolete now and is not available any where. the bandwidth is quite limited as well for some ics. i have a book in which some of these ics are mentioned like sad512 etc.

i will be buying the alpine 9887 Head Unit in a month or two.

what i want is to time align the mid bass drivers with the sub so that the the bass is upfront and integrate well.

i installed 2 way hertz components with an idq 15" d2v2 sub using a jbl amp in a friends car. the HU was alpine 9887. after tuning and TA, the bass was right up front.

i have the same comps(hertz energy 6.5" with an energy 15" sub and a pioneer amp(will soon be upgrading to audison) and a pioneer HU. the setup lack integration. so thats why this thread.

Originally posted by Eva
If you don't fear DIY DSP, check out AK7744. I discovered this IC yesterday while I was reverse-engineering a Honda factory head-unit to add a line-input. I was not expecting at all to find such an IC in a factory head-unit, it seems very smart and it's 24bit 48Khz. Since it includes 3ch-ADC, DSP, RAM, 4ch-DAC, I2S digital audio input and output and almost everything in a single package, it may be controlled with just a PIC micro, a display and a keypad.

BTW: Alpine headunits employ a similar solution.

will check it out. you guys will need to help me regarding the designing of the pcb. i can etch it myself though.

Originally posted by lms
alpine supplies some of the honda head units

same case here in india.

[Eva]

I'm new to designing audio DSP too. One of my future projects is to build something around AK7744 or a similar chip, but I have plenty of projects to finish before this.

Don't underestimate the usefulness of all-pass filters at low frequencies, they come quite handy when wavelenghts become long, for example 90 degrees of 160Hz is already over half a meter delay.

You may not need an Alpine 9887 either, you can already do great things with the old 9812 and 9813. Same for the 985x but with a worse interface. I don't know the newer 988x but I have been told that the interface is even worse in these. I hate 985x slider bar.

[clipto333]

i don't have any of the alpines you mentioned. im stuck with a pioneer 6800 mp3 HU.



i wish some one posts a circuit schematic for delay equalization.

[simken]

why not purchase an adjustable phase control from some company that builds subs?
wouldn't that work?

[Dan2]

this might be a stupid question, but if 90degree phase shift at 160hz gives half a metre delay - then what about connecting sub out of phase with mids??? will that give 1 metre delay??

[shagone]

Originally posted by Dan2
this might be a stupid question, but if 90degree phase shift at 160hz gives half a metre delay - then what about connecting sub out of phase with mids??? will that give 1 metre delay??

a lot of people do this and have good results with the subs blending in better with the mids. i'm not sure of the exact delay, i'll leave that to the experts here. I always try it and if i like the results i leave it.

i have even heard of flipping the tweeter phase or having one tweeter out of phase with the other but i have not played with that idea yet. besides for people with DSP there is no need to do this.

[Dan2]

just thinking now - if you adjust the sub to blend in with front mids, then it won't integrate with rear mids, so it will never be perfect (unless maybe delaying the front speakers )

I can't imagine hearing a difference with out-of-phase tweeters (especially not in a car)

i recently got a pair of Audiopipe tweeters for free from a friend of mine (and im a student so i NEVER say no to free stuff) and the wires it comes with doesn't even show polarity (both wires are black)

[Eva]

It's much more complex than all that. Some facts:

- Changing the polarity shifts phase by 180 degrees for all frequencies but it does not introduce any delay. Thus the degree of "time correction" achievable with polarity changing is very poor (but it's always better than nothing).

- A delay (either physical or electronically simulated) is a phase shift whose value is proportional to frequency. If a frequency band is to be delayed, phase shift must be made proportional to frequency in this band (like 90 deg at 100Hz, 180 deg at 200Hz, 270 deg at 300Hz, etc...)

- Each analog phase shifter stage only produces a good delay up to 90 degrees, then the delay vanishes for higher frequencies as phase shift is no longer proportional to frequency.

- Rear midbass, mids and tweeters are the opposite of sound quality unless they comprise an independent sound system fed with a specific "ambient" signal from a surround processor.

- Tweeter polarity does matter a lot, unless the distance from the midrange or midbass to the ear is very different from the distance from the tweeter to the ear. There is acoustic summing taking place in the listening position (and off-axis towards reflective surfaces), and the better the sum the more the polarity change is noticed.

[jol50]

In the old days we would always try the tweeters/mids/or subs out of phase. It affected the sound at the crossover point, depending on where they were at and overlap they had. If you had midbass boom running the subs out of phase cut it down as it canceled the frequencies at the overlap in the midbass area. The change in phase might also change the sub sound. If you had active crossovers you would move them around also when you did that.