How to reduce tape hiss while recording a cassette tape with digital data as source

Question

I bought a few Maxell cassette tapes, a portable Sony cassette tapes recorder (with the possibility to record also from an external microphone: in my case, a jack connected to the PC) and - as already said - I have a male-to-male jack to connect the recorder to the line out of the PC.

I tried to record something (a few FLACs on Audacity) after having set the volume loud enough but not so much that it would make the recording sound distorded and, while it seems that the audio is correctly recorded, I can hear a lot of tape hiss. By a lot, I mean quite a lot more than the one I can hear in old tapes I have at home.

Are there any tricks (in Audacity or not) to reduce it? Thank you very much!

Answer 1

Welcome to the 1970s!

First, make sure you've got Dolby noise reduction on, if it's available. It boosts the treble when recording and turns it down on playback reducing the hiss and improving the S/N ratio.

If you don't have Dolby your best bet is to simulate it manually by increasing the treble during record, etc.

On my JVC tape deck I got good results by recording with Dolby and playback without.

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Line out to mic in

Re-reading your question I suspect that you are feeding line output from your PC to mic input on your cassette recorder. If that's the case then the problem should be easily solved.

You have experimented and found that by reducing the signal level from Audacity that you can avoid distortion on the mic input. While this attenuates the signal it leaves the noise from the sound card output unattenuated. This means that your signal to noise ratio has decreased drastically.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Stereo line to mono mic attenuator.

We can fix this simply. Send the signal out of the PC sound card at full volume. This will restore the signal to noise ratio. Then attenuate the signal and the noise between the PC and the mic input.

I would guess that the PC sound card will put out about 1 V p-p an that the mic input will be about 10 mV p-p. Try attenuating by 100:1 and see how you get on.

How it works

Take the left channel: R1 and R3 form a voltage divider given by the rule, \$output = input \cdot \frac {R3}{R1 + R3} = input \cdot \frac {100}{10000 + 100} = \frac {input}{100}\$ approx. It's called a 100:1 voltage divider.

The right channel works the same.

How to make

• Find some suitable resistors. Exact values isn't as important as the ratios between them. Cost is about 20c.
• Cut your 3.5 mm patch lead and identify the cores.
• Connect up the ends of the wires as per the schematic. If you don't have a soldering iron then use a screw terminal strip.

Answer 2

Does it record hiss (noise) if you have no signal from the PC or if you disconnect the PC end and short the contacts and record a 0 level? It would be good to see if the PC is adding so much digital noise on that it is getting recorded. Common mode noise due to digital circuitry may not be audible from the PC but may get coupled capacitively or through a ground loop and then amplified in the recording circuit.

The quality of consumer recorders has not improved after the 1980's and specifications are pretty loose.

You may also want to double check the levels (voltage) and inpedance matching between the PC and the recorder. LINE OUT from the PC may be better suited to a LINE IN on the recorder. If it only has a MIC IN then it may be too sensitive and you may have to atennuate and try and match the impedances.

Answer 3

while it seems that the audio is correctly recorded, I can hear a lot of tape hiss. By a lot, I mean quite a lot more than the one I can hear in old tapes I have at home.

Pre-recorded music tapes are produced on high quality tape decks. Your tape recorder is a budget consumer model which was only designed to record voice.

The main reasons for higher noise are:-

1. The MIC input is set to a low level to suit the low output of a microphone, so internal amplifier noise will be higher relative to the signal than a line input would be.

2. Tape bias is achieved using a permanent magnet. This creates higher background noise than AC bias.

Reducing the Line output level with a resistor divider may reduce noise coming from the PC, particularly if you have to reduce the volume level to avoid overloading the recorder. However it won't reduce noise produced inside the recorder. To fix this you would have to seriously modify the recorder's internal circuit (eg. bypass the microphone preamp, add a tape bias oscillator).

Answer 4

The hiss could be caused by a lot of factors. first of all being a mic input feeded with a line input means that has an high amplification, coupled with a source with different impedance that a regular mic. So as some other poster has noted an attenuator could improve the results. Another factor is that some RF noise could interfere and be added to the recorded signal. Thisrd is that if you are using a cheap cassette recorder could be intrinsecally noiser in record than in play. It's easy to find secondhand cassette decks for separares hi fi at a low price and these have also normally Dolby noise reduction and a variable record level.