Digital Baby Monitor

Introduction

We have two young children in a small town house in the UK. We use wireless baby monitors so that each child can sleep in their own room, with the door closed, and not wake each other up if either cries, while we can hear both. (Well, ideally.) We also use these monitors to hear our children if they are sleeping in other rooms (or even a tent in the garden) when visiting friends and relations, sometimes for big events such as parties which involve a lot of noise.

We have tried several 50 Mhz analogue monitors and found all unsatisfactory in some respect. By contrast our 1900 MHz digital cordless phone, which conforms to the DECT standard, performs flawlessly in the same conditions. Some DECT handsets, in pairs, can already be used as baby monitors. However, I believe that there is a gap in the market for a dedicated DECT-based baby monitor which no manufacturer yet provides.

So if you are a baby monitor manufacturer, why not make something that meets the requirements detailed here. This won't happen in time for our children, who are outgrowing the need for such things, but may help other parents in the future.

Note: a reader of this page has kindly sent me (27Apr2002) a link to this Philips product, which seems to be very much what I was after. In fact there seem to be several models now on Philips' consumer electronics site. Bravo!

Characteristics of 50 MHz Baby Monitors

Cheap, from about GBP 30.
Analogue sound encoding (AM or FM).
Frequent problems of noise and dropouts depending on precise positioning of transmitter, receiver and other objects (e.g. parents!) occur even at only a few metres from the transmitter.
Easy to be overheard by neighbours with similar devices (potentially embarrassing).
Bulky, heavy packaging common.
Usually have choice of only two radio channels.
Fail quietly if batteries run down.
At least one FM monitor has no indication of reception failure.
At least one monitor is marketed as "digital" but in fact only transmits temperature information digitally, causing yet more noise on the analogue sound channel.

Characteristics of DECT Digital Cordless Phones

Moderately expensive (about GBP 80).
Long range, easily working anywhere in and around a large house.
True digital sound encoding.
120 logical channels, sufficient for any number of neighbouring households to use a channel each continuously and simultaneously.
Sufficient encryption to prevent being trivially overheard.
Small lightweight handsets available, recharging on base station.
Visual indication if out of range or base station has failed.

Requirements List

Good Useful Range
The monitor must be usable between any two rooms in a large house (or small hotel) without requiring special positioning or a mains supply at either end. This may (for example) involve a distance of 30 metres and attenuation by three ceilings and five walls. A clear line-of-sight outdoor range of 100 metres or more is required for camping.
Channels
To work well in a crowded street or block of flats which may have many young families using monitors, at least 10 completely distinct logical channels are required. It is not sufficient to share codes on one or two frequencies (though time-sliced channels are fine).
Interference
The monitor must allow the parent to hear quiet sounds at the child end (shuffling, breathing) and produce no audible noise aretefacts of its own over the full distance range described above.
Privacy
The monitor must not transmit using a method or protocol which allows it to be trivially decoded by an unintended receiver. (This is just to avoid the embarrassment of neighbours overhearing private conversations by accident. A highly secure protocol is not required.)
No Low-Sound Cutout
Some existing monitors transmit only when the child is producing sufficiently loud sounds. However, it can be reassurng to hear quiet sounds (breathing and rustling), e.g. if a child is unwell or the parent particularly paranoid. So any low-sound cutout should be optional if present at all. It does have the potential advantages of reducing power consumption and the utilisation of public bandwidth. If it exists, it should preferably be switchable from the receiver (though this requires two-way communications).
Not Working Alert
If the transmitter has failed, been switched off, has the wrong channel selected, is out of range or has its signal obscured by radio noise, the receiver must indicate this to the parent using both visual and audio cues. Similarly the receiver should alert the user before powering off if its battery is exhausted rather than failing quietly.
Visual Sound Alert
The sound level produced by the child must be displayed using a series of bright lights or similar. This is useful in very quiet circumstances (when the disturbance of a child crying would be unacceptable) and in very noisy circumstances (when crying might be drowned out by music at a wedding reception, for example). Alternatively (or additionally), a vibration alert would be useful.
Power Supply
To be practical at all, the receiver must run for many hours using only an internal supply. For use in camping or staying away from home where mains sockets may not be positioned suitably, the transmitter too must be able to run (for many hours) on an internal battery supply. This also maintains service during any interruption to the mains supply. It is optimal to have both ends have the ability to take disposable batteries (good for camping or use in countries with incompatible mains sockets yet compatible radio licensing), yet be rechargeable. Some mobile phones already achieve this by using AA NiCd cells as their rechargeable supply while allowing substitution of those by alkaline AA cells.
Small and Light
The receiver should fit unobtrusively into a pocket for mobile use. Ideally it should be no bigger than a small mobile phone. Many existing monitors are very large compared to modern electronic devices such as DECT handsets.
Belt Clip
The receiver needs a belt clip to attach to the clothing of the parent for mobile use.
Rugged
Both the receiver and the transmitter are likely to be dropped, sat or trodden on, and exposed to humid environments and water splashes. The monitor must operate normally given such exposure to environmental stress.
Snooze Funtion -- Optional
During repeated visits to a child obstinately refusing to go to sleep, it would be nice sometimes to have a button which would silence the receiver for a few minutes and then have it switch on again automatically!
Not Required
Two-way operation might be useful sometimes, especially with older children, but is not worth paying a hefty battery life, size, weight or price penalty for. The night-lights on some transmitters are pretty much useless and should be deleted to reduce cost and improve packaging and performance. A temperature reading of the child's room is not very useul either.
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