Being a curious type, I've run some tests on the 404 audio quality. Not because I'm trying to find fault, but just to see what it's doing.

I see some think the 404 has a compressor built in, but I'm not so sure, except the built in mic might have some kind of AGC.

What I did find was that above 2/3 of the volume knob, the line out starts to get dirty . At full volume, distortion is around 10% - that's guitar amp territory! Under 2/3, it's as clean as most things it'll be used with.

Somethings whacky with the frequency response too. At high volumes, low bass and the top end drop off, while at a low volume, its more level. Some kind of loudness circuit in the volume knob perhaps, like some car stereos have? Otherwise, frequency response is pretty level most of the way.

Stereo tracking of the volume knob is a bit uneven, that is, I couldn't get a mono sample to show exactly the same level on L&R of my test meters except at full volume. That volume control is a pretty cheap one.

Recording (line-in, normal quality) is really fairly clean.

The audio data compression the 404 uses is actually pretty good, it adds a bit of distortion but doesn't really touch the frequency response.

I wondered what the compression is. If it was mp3, they'd have to say so because mp3 is licensed. But Roland have their own data compression ststem called R-dac. Anyone who's owned some Roland VS or Boss BR recorders might be familiar with R-dac. The 404 normal quality tests exactly the same as the VS recorder MT1 quality.

I can't say for certain, but the Lo-fi might be exactly the same audio with the lower 8bits left out. The samples are about half the size of normal quality. Lo-fi rips most of the upper frequencies out anyway, with a some HF stuff left at the top end.

I might have to open my 404 up for a good look inside. Like I said, I'm curious, and if you can't open it, you don't own it. Maybe there's room for some modding?

Ive been filtering my tracks through the 404, even without any master effects on. It seems to soften the highs abit and give abit of compresson and distortion thats rather pleasing to my ears

yeah i've noticed distortion occurring when the volume's way up...kinda sucks. i guess we just have to aim to never have the volume too far up.

interesting to read your findings though. you should definitely open it up, see if it's easily moddable. would be great to be able to add a pitch control

man i eq that shit wit tha bass up. Then compress that shit....'My speakers be rumblin too much bass.

Well, I've had a look. Plenty of cheap amp chips in there, but I don't think it would solve anything just by upgrading them.

BTW, I'm not suggesting the 404 sound should be clean - I just wonder if the dirt is deliberate and very cleverly designed, or a side effect of cheap electronics.

OK, there's 3 circuit boards.

A big one, most of the area of the box that holds the buttons, knobs and displays.

One half that size at the CF card end with all the digital and audio circuits on it - the system board

A small one at the back that carries the sockets and switch.

The boards themselves are very good quality and the cabling between them is socketed. The plastic case does have some steel screening plates trapped inside, but it isn't completely screened.

The system board is where all the action is. The analog circuits are all down one side, but it's no surprise the mic amp picks up system noise being on the same board as the digital stuff.

I think it would be fairly easy (wiring wise) to add a seperate headphone volume control, but I can't see anywhere with enough space to mount one!

The internal mic is the tiniest thing I've ever seen, mounted right on the board, so no wonder that mic picks up handling noise so easily - there's nothing to isolate it from direct vibration. May be possible to move it onto some kind of foam mount?

The volume control is an ordinary stereo job. Nothing special except it is a 5 pin control whereas most are 6 pin - not so easy to find a replacement. One with detents would be nice.

There's one chip I saw, a 4066 (a signal switch), that's known to distort with hot signals - maybe that's mainly responsible for "that sound" - but I can't discount the possibility there's a fixed digital effect in the signal path.

I'll have to double check - I couldn't read the number too well, but the digital/analog converter "codec" chip might be an AK4552 which is 24bit as the 404 spec' claims.

What kind of pitch control is needed? Old style tape speed or pitch change only, retains speed.?

tape type, the kind u find on a turntable. Basically a "tuning" function that doesnt sound horrible

Jim, are you able to snap a few photos of what lies inside our favourite box?

thanks

edit: good to read your thoughts on the circuits...i know very little about electronics and would be clueless if i opened mine up

OK. tape/turntable speed. Only way I can think this could be done is replace the 44.1K sample clock - If I could find it - with a variable one. That's how the vari-speed works on a VS recorder.

Pic'? - only camera in the house at the mo is shit at close ups. might try stills off the camcorder - stand by.

There really isn't a lot of space in 404. But the analog circuits are up for grabs. You could...

Replace the amp chips with low-noise types. Might especially be worthwhile with the mic amp.

Take the internal mic off the circuit board and connect by flexible wires and mount in some foam, so it doesn't directly pick up case noise so bad.

The mic amp is on the opposite side to the audio circuits and has to pass under all the digital stuff on the circuit board tracks - cut those tracks and re-route using screened audio cable around the digital areas.

There's some mixing going on in the audio amps, like between the digital DAC output and the direct analog input. Then there's the record switching between line and resample and mic inputs. If there was space for extra controls, you could have mixer level knobs for each input and output and even an overall stereo balance control - but I don't see a way to add a pan control for mono samples - that would have to be done in a software change.

Space - that the big stopper. I wonder if thumwheel controls like on small radios will go on the side?

Maybe, on the left of the display, there's just room for a multisegment stereo LED level meter - and a switch to pick if it shows input or output level.

Maybe, you could have a dimmer for the all the 404's lights. Easier on the eyes in low light and probably extend battery life a fair bit.


Not promising I can or will do any of this (there goes a 3 year warranty!). But am enjoying pondering the possibilities. All ideas welcome!

Cheers
Jim

OK, try this link...
http://hometown.aol.co.uk/AshdaleStudio ... ernals.JPG

This is a basic view of the underside with the bottom cover removed - six screws and carefully pull out the 2pin plug from the battery cradle.

I've put some text on it describing general areas. Am working on a closer view of the main board with text pointing out more specific items - when I've figured out what they do.

cheers
Jim

Hey thanks for the pic. It is really tight inside, but I guess any mod stuff maybe can be placed between the main board and the analog I/O board.

really nice job you are doing here! very intrestning to read!

cheers for the photo Jim

i'm really interested by your findings and speculations. seems to be some prospect of modification. the thumbwheel control idea is clever. seems pretty challenging to replace the sample clock though, seems we might have to forget about pitch control

have you checked the circuit bending thread? viewtopic.php?t=303
iG9 has some experience and may have some ideas



i'm wondering what the purpose of those analogue elements is? what do they control, what signal passes through them?

and in terms of the 404's Effects, are they just on a microchip? or is there some interaction between the analogue circuits?

Well, I'm working thru the analog stuff to see what each chip does. Aim is to produce a signal flow diagram. So far I've managed to identify and get the datasheets for these chips. I can say that as yet, I've found nothing special -no compressor or anything like that - the chips are either amplifying or mixing signals and there are a couple of stages using single transistors.

Re vari-speed, I think the sample clock comes from a large Boss chip, probably the FX. It has a crystal by it that regulates the clock. You probably can mess with this crystal, sometimes you can "re-tune" them with a variable capacitor wired across it (the capacitor equivalent of a potentiometer). Trouble with those is, it upsets them just to go near them with your fingers, and you can never be sure if you've returned it to the correct rate.

Re Circuit bending. That's a good thread, but I've always thought those modding techniques work best with stuff that generates sound mostly in analog circuits. All we have in the 404 to mess with are the circuits which amount to a basic mixer. We can mod this by inserting extra circuits, tapping off extra outputs, fit extra inputs, replace or bypass parts to use better ones.

The mod I'm most keen on at the mo', is to fix the mic pre. It doesn't have enough gain and picks up too much noise as it is. I've a feeling it would be easier to completely bypass the existing amp with a new circuit. With a good mic pre, I think the 404 would make a usable field recorder.
You cannot easily modify what it does on the digital side - it's not impossible, someone hacked the I-pod to unlock it's line-in recording - but it's not something I'm capable of.

OK, I've posted a signal flow diagram here...
http://hometown.aol.co.uk/AshdaleStudio ... flowRO.pdf

It's very simplified - it doesn't show seperate stereo paths because they're just duplicated anyway. A "2" on a line means stereo.

Cheers
Jim


Worth noting tho' that nearly ALL the mixing is done in digital, except for the Mic-in/line-in mixed together in analog. That input mix then goes straight in to the ADC to digital, and IF the Ext Source is on, come back out of the DAC already digitally mixed with anything else (samples/FX) and on to the outputs.

I uploaded a new diagram.
Please treat it as "work in progress"!

I just found what looks like another feed from the mic amp to the output - which doesn't make sense to me yet.

The circuit board is way more complicated to work out than I first thought - it's multilayer, that is, there's circuit tracks inside the board as well as top & bottom. It really is very well made though.

I've been concentrating on the on-board mic, since that's something I'd like to improve. looks like the mic capsule's glued on - probably couldn't remove it without breaking it. The mic amp chip, a 4570, is really quite a good quality audio amp - not the best, but the problem is the analog stuff runs on only 6volts, and there's no better chip I could easily get that works that low, most need at least 10volts. The best I could easily do is add another amp chip after the existing one, so the existing one doesn't have to have it's gain turned up so much to get a decent level to record ambient sound with. But there's still the problem that the internal mic picks up vibration thru the case too well.

I did a test recording of the mic amp with the mic trim control on max and put in a dummy 1/4" plug so I only got the amp noise. Running off either battery or AC power, the noise recording was about -48dB - NOT GOOD! -48dB might not seem a lot, but you should (in theory) get about -72dB with 16bit recordings. I'd be happier with better than -60dB from a mic amp. Electronic stuff always generates some noise of it's own, and amplifier circuits go and amplify it more, so you can't expect total silence.

Sooo, I though, how about an add on? And stereo at that.

I'm picturing a little rectangular box that plugs on over the back of the 404, directly onto the line-in jacks. Each end of the box has an electret mic for Left & Right. Power comes from it's own 9v battery, or, if I add a new jack on the 404 backplate, off the 404 battery. The box could have a vol control for each mic and possibly a little 4 or 5 segment LED level meter.