What's the best solution for audio loading on a 48k?

Smith · May 2016

I have three 48k Speccies. All have been recapped and AV modded, but only one (an issue 4A) is currently loading via Teezix. (the other two (issue 3 and issue 6) have a go, and flash their borders a bit, but no dice)

Is there any kind of simple mod that allows a Speccy to load reliably from more modern audio devices, or am I looking at inserting some kind of mono amp between the Speccy and the audio source?

Spec-chums, how do YOU load?

FrankT · May 2016

Most phones and android devices in general, don't output loud enough for the spectrum.

Not tried Teezix app before, but if it has a boost volume option, try it.

It's interesting though that your issue4a does work. Would be good to know why.

Edit: Just looked at Teezix. Have you done this with the other model Speccy's?
"Use the built in test tone generator to get the sound level just right for your Speccy."

GReW · May 2016

Its unlikely you'll get a loud enough signal from a tablet or phone headphone socket without some sort of amplifier.
Personally I use an old mono-datacorder and one of these:

Smith · May 2016

Yeah, I'm wondering that myself. it's quiet, but apparently just loud enough. I'm leaning towards the theory that one of the resistors may be out of spec.

I noticed in the schematic that there's a slight difference in the value of capacitor C37 between the 3 and the 4a but that doesn't explain why the Issue 6 still has issues, aside from the fact that it's a more substantive redesign than the issue 3 or 4a. The schematic around the ear plug is virtually identical in the issue 3 and 4a.

johnsmith · May 2016

Speccy Tape app has been doing very well on a gen4 iPod. If you have Interface 1 then loading snapshots from PC via serial is also an option.

Smith · May 2016

In the long term I'll probably get an SD solution, but I don't want to neglect the mic & ear ports since they're both directly tied to the ol' beep-box.

Your Spec-chum · May 2016

As I've mentioned in a few similar threads, it's important to use the correct cable to connect modern device's to Spectrum. Smartphones, Ipods, MP3 players etc have a stereo(2 channels) output. Spectrum's EAR & MIC sockets are mono(1 channel). If you use the original tape leads supplied with the Spectrum, which are only wired mono, and plug them into a device with a stereo output, it will short the right channel to ground. This will cause a load on the device's amplifier and reduce it's output, and could damage the device.

Smith · May 2016

Unfortunately my original mono cables are long misplaced. Instead I use standard stereo cable and stereo to mono adaptor at the Speccy end.

Smith · May 2016

FrankT wrote: »

Most phones and android devices in general, don't output loud enough for the spectrum.

Not tried Teezix app before, but if it has a boost volume option, try it.

It's interesting though that your issue4a does work. Would be good to know why.

Edit: Just looked at Teezix. Have you done this with the other model Speccy's?
"Use the built in test tone generator to get the sound level just right for your Speccy."

Yeah. Unfortunately the devices at my disposal (s3 & core prime) just don't seem to have the oomph to drive the volume level high enough through the headphone jack.

ingo · May 2016

A simple way to increase the voltage for the tape input of the Spectrum (EAR) is to insert a transformer with a ratio of 1:3 between audio device and ZX Spectrum for example that here: produktinfo.conrad.com/datenblaetter/725000-749999/739614-da-01-en-UEBERTRAGER_1_55V__540MH.pdf

Ingo.

textvoyage · May 2016

An old Nokia N78 worked for me - I converted TAP files using Tape2Wav, copied the WAV files onto the phone and played them into the Spectrum's ear hole. I tried some other old phones but they were too quiet.

RobeeeJay · May 2016

Although I've not done it yet, at some point I keep meaning to build something like this which fits inside the case, then you could feed it any signal you like within reason and it would amplify or reduce as appropriate:

ingo · May 2016

Hi RobeeJay,

this circuitry is working well for output levels of some ten mV (it was used in tape recorders for automatic level control). For higher voltages it does not work. So if you plan to use such circuit you have to place it before an amplifier that amplifies the signal to a level of > 1 V as needed for the Spectrum.
If you look at the video in fullscreen you can see that the peak to peak amplitude of the output signal is about 45 mV - much too low for the Spectrum.

Greetings Ingo.

hikoki · May 2016

Dunno if loading via bluetooth reveiver as explained here: http://www.zxuno.com/forum/viewtopic.php?f=25&t=327
could make the spectrum hear louder

RobeeeJay · May 2016

ingo wrote: »

this circuitry is working well for output levels of some ten mV (it was used in tape recorders for automatic level control). For higher voltages it does not work. So if you plan to use such circuit you have to place it before an amplifier that amplifies the signal to a level of > 1 V as needed for the Spectrum.

Yes, it would need some altering, also his circuit runs off 12V. But the principle is what appealed to me, no more worrying about levels, however in practice if the levels are way too loud or quiet it might not adjust quick enough, would be an interesting option though.

I haven't looked but ebay may even have ready built modules...

ingo · May 2016

RobeeeJay wrote: »

however in practice if the levels are way too loud or quiet it might not adjust quick enough, would be an interesting option though.

I think this is no problem as too high level is not the problem. The signal is digitized in the ULA. Only the timing of the flanges of the signal is used to recreate the recorded bits.
Therefore the level should be in a (relatively wide) range above a minimum value to be good for loading.

Ingo.

Pegaz · May 2016

I am using for several years smartphone Samsung Galaxy S+, with PowerAmp media player.
Mostly in wav format, with inverted polarity:
https://translate.google.com/translate?sl=es&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http://foro.speccy.org/viewtopic.php?f=8&t=1987&edit-text=&act=url

1024MAK · May 2016

You have to keep in mind, that until you get to the digitizing circuitry inside the ULA, we are taking about an analogue system. Component tolerances alone cause slightly different sensitivity between different Spectrums with the same board revision and the same ULA revision.
Also there may be slight differences between different issue boards. But the biggest difference is between different versions of the ULA, and maybe different batches of the same ULA version.

The Spectrum needs a fairly large AC input voltage. Modern earphone audio outputs are normally designed for modern earphones. They can supply the current required for these sensitive earphones, but the output voltage is limited, in order to limit the power. Also the internal amplifiers run from low voltage supplies (3V to 4.5V), unlike the 6V to 12V that were found in the old mono cassette tape recorders.

The available output voltage makes a big difference to the Spectrum. If the voltage is too low, the ULA does not see the positive peak of the waveform, or it only sees the peak at erratic intervals as the AC signal level varies a bit. If the signal level to the Spectrum is too low, the only solution (without modifying the Spectrum) is to amplify it.

Smith wrote: »

Unfortunately my original mono cables are long misplaced. Instead I use standard stereo cable and stereo to mono adaptor at the Speccy end.

You need either a custom lead where only one of the two stereo channels goes to a 3.5mm mono jack plug. Or an adaptor where the two stereo channels go to two 3.5mm mono jack plugs, one for the 'left' channel and one for the 'right' channel. By convention, the left channel is used for mono, but it does not really matter.

Mark

Smith · May 2016

The phone is in Mono mode, therefore left and right channel should be the same, and since it does work on at least one system, I assume the audio cable is sound and the output merely too quiet.

As far as I can tell, the only major difference in schematic between those revisions is that the value of C30 is halved (47nf in the Issue 3, vs 22nf in the Issue 4), but I don't know enough about analogue electronics to say what difference that may make.

I might experiment with altering it to see if it makes a difference. Already made one mod to make it work better with modern equipment. In for a penny, in for a pound at this point. It seems unlikely that it's going to be used with vintage equipment to play audio tapes in the future, after all!

1024MAK · May 2016

But the outputs of the left and right channels go via different electrical paths, so there will be slight differences in timing and level. 3.5mm mono jack plugs/sockets and 3.5mm are not electrically compatible with 3.5mm stereo jack plugs/sockets. Either a channel is shorted out, or proper contact is not made.

Look at a 3.5mm mono jack plug and a stereo jack plug side by side. The contacts in the mono socket have a wide area to connect to the two contacts.
Now when a stereo plug goes in a mono socket, often the contact in the socket shorts across two of the three plug contacts :(
With a mono plug in a stereo socket, two contacts will connect with one contact on the plug.
Because the ground/0V/common is the sleeve end (furthest from the tip), this can cause one channel to be shorted out completely.
Or the left channel can be shorted to the right channel.

47nF is less attenuating to mid and lower frequencies compared to 22nF. The exact amount is frequency dependent and also determined by the other components in the circuit.

Mark

TheTurnipKing · May 2016

The Mono adaptor I mentioned seems to short left and right together, according to my multimeter.

FrankT · May 2016

When left and right are shorted together, you get phase cancelling. It makes parts of the signal invisible, or cancelled out. When two frequencies are played against each other, this is what happens. It's the tech behind noise cancellation headphones, that remove background noise. First developed as something useful for airplane flights to remove the background noise.

1024MAK · May 2016

TheTurnipKing wrote: »

The Mono adaptor I mentioned seems to short left and right together, according to my multimeter.

Which is fine when feeding a mono output to a stereo input (the mono feeds both left and right channels).
But no good for anything else :(

This is more like it: eBay item 360729306414

3.5mm STEREO Plug to 2 x Mono Plugs
BRAND NEW
1 Metre Cable
3.5mm STEREO Jack plug to 2 x 3.5mm MONO Jack Plugs
Audio Splitter Converter Lead

I *think* someone here has bought one of these, but I forget if they said it worked as expected.
You could contact the seller to confirm if each mono plug is the left and right channels on the stereo plug.

Mark

ingo · May 2016

In some cases (if you use the stereo signal of a cassette player) it could be better to use only the right channel. If the tape head is not mechanical adjusted in exact the same way as the recording head then there is a phase difference between the left and the right channel of a stereo signal. In that case merging both signals results in a loss of high frequencies or in bad flanges of the signal. The right channel is the (mechanical) inner track on the tape (lower risk for drop outs). So it is better only to use this channel for loading signals into the computer. If the stereo signal is coming from any digital device as computer, IPOD MP3 player or similar then it also not better to use the sum of two channel then to use only one channel.
That's why I use a cable that devides a stereo signal from a 3,5mm stereo plug two two 3,5mm mono plug. This way I can decide which channel I like to use.
Ingo.

Alessandro Grussu · May 2016

I have been feeding TZX files to my Spectrums for years by using different techniques and sources:

- TZX played from the PC with Tapir;
- MP3 files converted from the original TZX with WinTZX and played from a multimedia reader;
- TZX files converted as 44100 Hz, 16-bit stereo WAV files burned as audio CD tracks.

The first two methods require an external amplification in order to work correctly. A PC loudspeaker with an EAR socket will do. Take a look here, here and here.

The third method won't require any amplification as far as I could see by myself.

For the record, car stereo adapters are what I use to load software on the +2A; there is no need to keep the PLAY key pressed on the Datacorder: read here if you wish to know more.

Peter Mulholland · May 2016

There is no A/D as such inside the ULA.. instead, it is just a threshold comparator - actually I believe it is just the input of a logic gate that "just happens" to be triggerable by a loud enough audio signal. In my experience, the best thing to do is make a comparator circuit (as shown by JoulesPerCouloumb's video) which translates the signal to a clear 5V logic level. This is exactly what the +2 tape deck does.

Peter Mulholland · May 2016

If you're paranoid about blowing things up by having the volume too high:

I've been using the above circuits just fine with a portable MP3 player that barely produces 200mV of signal. It should certainly work with any mobile phone.

What's the best solution for audio loading on a 48k?

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