Personal Video Recorder

In 2012 the Raspberry Pi became available. The model 3B provides:

• a 4-core processor,
• USB-2.0 ports,
• Ethernet & WiFi,
• hardware-accelerated h.264-decoding.

This is sufficiently powerful to run kodi, & is quite inexpensive. Many people have pioneered such solutions, but I've detailed my specific solution & conclusions.

Case

The case is built from acrylic.

• This typically comes covered by protective plastic, which shouldn't be removed until completion. 3 mm was selected so that the grommets used to mount the HDDs could threaded through the width.
• The A3 acrylic sheet is scored & snapped over a straight edge (which takes considerable force), into two A4 halves; one could merely buy A4 sheets (which I did for the bottom layer), but that's slightly more expensive. The sheets can then be clamped together to reduce the effort of subsequent processing & to force alignment of those holes they share.
• The corners are hack-sawed & then ground to a radius of ≈1 cm; large washers can be clamped either side, to form a guide for the file. This is largely aesthetic, though my sheet of acrylic arrived by post with one corner pre-mashed.
• Various holes were drilled using HSS twist-drill bits.
  • For accuracy a drill-stand was used, after piloting each hole by hand using a pin-vice & a 1 mm drill-bit (which easily bites into the relatively soft protective plastic).
  • Acrylic tends to ride up the thread of a twist drill-bit, & since it's brittle, the consequences are typically catastrophic, so each hole was progressively enlarged 1 mm at a time.
  • Excess heat can melt & discolour the acrylic, so a low drill speed was used & also peck-drilling for wider holes.
  • To reduce chipping, wood was placed beneath the acrylic to support the perimeter of the exit-hole as the drill-bit emerges from the far side.
• A square-section acrylic bar is bonded to the lower side of the top acrylic sheet, between two holes from which both DVB-tuners are suspended using a cable-tie.

The horizontal acrylic sheets are connected to form two tiers, by vertical aluminium tubes at each corner, through each of which a threaded rod is inserted coaxially. Penny-washers are used to spread the load from the tightened threaded rods, to avoid cracking the acrylic. The aluminium tubes don't reliably centre around the threaded rod (as can be observed in the photos), which was partially resolved by inserting M3 × 9 mm washers into the base of each. The length of the tubes separating the upper tier, plus twice the thickness of a penny-washer, should equal the 70 mm width of the HDD plus the thickness of the grommets. The length of the tubes separating the lower tier is less critical, but must exceed the combined height of the battery & crimp spade-sockets (the descending spade-terminals from the battery-switch above are offset from the battery). In either case a pipe-cutter is used to achieve perpendicular ends; using a hack-saw & file is considerably more laborious.

The threaded rods are capped by acorn nuts to shield the sharp ends; these were added after the photographs were taken. Rubber feet must be threaded onto the lower end of the threaded rods, since otherwise various protruding bolt-heads & cable-ties would prevent the case from sitting horizontally.

Computer

cat >>/storage/.config/modprobe.d/blacklist.conf <<-EOF	# Disable RPi's weak internal WiFi.
	blacklist brcmfmac
	blacklist brcmutil
EOF

The Raspberry Pi is attached above the middle acrylic sheet using 12 mm long standoffs to enable subsequent extraction of the µSD-card using one's fingers rather than tweezers. The upper end of the standoff must be female since there's insufficient space to tighten a nut; the lower end can be either gender.

There was apparently no need to attach a heat-sink to the Broadcom BCM2837 SoC. While recording two high-definition channels & watching a standard-definition recording, the temperature rose from a quiescent 39 C, to 57 C; well below the 80 C at which throttling occurs.

Though the Raspberry Pi has 4 USB-2.0 ports, it only has a single bus, limiting the data-rate to a theoretical 480 Mb/s. In the UK, each DVB-t2 tuner requires about 40 Mb/s. Though each HDD has a SATA 6 Gb/s interface, it won't be required to write any faster than both DVB-t2 tuners can simultaneously receive. So if recording two HD channels while watching a third, the total requirement (200 Mb/s) is still within what might be expected from the USB-2.0 specification, provided that the Ethernet-interface (implemented on USB) isn't being hammered for any unrelated reason.

mount -o 'remount,rw' /flash;	# This file-system is normally mounted read-only.
echo 'dtoverlay=i2c-rtc,ds3231 # For realtime clock.' >>/flash/config.txt;	# Append a line to the config-file.
reboot;
hwclock -r	# Confirm the time.

Storage

The selected distribution is read from a µSD-card; I used a 32 GB (though 8 GB would have been sufficient) Samsung Pro Endurance, its read-speed easily exceeds the 25 MB/s speed of the card-reader (minimising boot-time), & claims significantly higher write-endurance. For installation, either follow "these instructions", or install NOOBS & select the required distribution.

Settings → Add-ons → Install from respository → LibreELEC Add-ons → Program add-ons → BTRFS Tools → install
Settings → LibreELEC → Services → cron → Enable cron

crontab -e	# Edit root's crontab.
0 2 1,16 * * $HOME/.kodi/addons/tools.btrfs-progs/bin/btrfs scrub start /var/media/Video

echo 'export EDITOR=vi' >>~/.profile;	# Specify your favourite editor.
. ~/.profile	# Source the new profile.

Since kodi is not just a PVR, but a media-player, any audio files can be read from a USB flash drive. To avoid corruption of the file-system on power-loss (especially if you don't opt to build the UPS), ssh into the Raspberry Pi & issue the command:

echo "mount -o 'remount,ro' device" >>/storage/.config/autostart.sh	# Remount the file-system read-only.

RF

Two DVB-tuners were used, to permit simultaneous recording. I used a Geniatech MyGica T230 & a PCTV Systems tripleStick 292e, both of which worked seamlessly. DVB-T (Freeview in the UK) was piped to them via an RF-splitter. Initially I used a cheap four-way splitter, which at best quarters the signal-power available to each output, resulting in a signal that was unacceptably weak, so I replaced it with a higher quality two-way inductive splitter.

The assembly of RF-components was hung beneath the top acrylic sheet of the case, taking care to distance them from the GPIO-pins rising from the Raspberry Pi; connecting the 3.3 V pin to the 5 V pin, or shorting either to ground, is a great way to blow the Raspberry Pi.

USB-hub

The 7200 RPM Hitachi Z7K500s selected for storage of video files, require only ≈2 W each, but can draw a rather high 5.5 W on start-up (the 5400 RPM version is little better).

The power-requirements of both the HDDs & two 0.5 A DVB-tuners, exceed that which can be delivered from the USB-ports of the Raspberry Pi, so they're connected to a four-port powered USB-hub; I selected a hub which conforms to USB 3.0, not because of its speed (since the Raspberry Pi to which it is connected only supports USB 2.0), but because it makes 0.9 A available to each device, which is necessary for the HDDs.

I selected a 15 W Atolla 204u3. Input-power at 5 V is delivered via a barrel-connector, allowing both this & the Raspberry Pi to be powered using one buck-converter. The various USB-cables can be merely USB 2.0, though one might want to confirm that their power-wires are of adequate capacity; I just used USB 3.0 ones. The orientation of the USB-ports avoided the need to twist thick the SATA-USB cables through 90°.

Having allocated all of the ports of the USB-hub to high-power devices, I connected the USB flash-drive containing the audio files directly to the Raspberry Pi; this is tolerable, since its power-requirement is only 0.2 A. If one decided to replace the Raspberry Pi's WiFi with a higher-power adapter, then a seven-port USB-hub would be a better choice (though they typically require a 12 V power-supply).

Back-powering the Raspberry Pi from the USB-hub also, bypasses protection & is generally deprecated, so it retains its own independent power-supply.

UPS

Though the maximum operational power-requirement is only ≈20 W, the mains-plug has a 3 A fuse to account for the higher current on start-up.

The 230±10 V AC mains is converted to 12 V DC. Since the selected PSU is rated at 60 W output but never exceeds 20 W, it only reaches 38 C, & since it doesn't have any cooling-holes beneath, it is mounted directly on the base acrylic sheet with two M3 hex socket bolts.

This is used to charge a sealed lead-acid battery, which returns the charge should the mains voltage be disconnected for any reason. Since the PVR typically takes ≈18 W, the 2.1 Ah 12 V battery can last for almost 1.5 h. This battery is positioned to ensure that its terminals are in no danger of contact with the bolts for the HDD above.

This intermediate DC voltage is then stepped-down to supply both the Raspberry Pi & the USB-hub; both of which can tolerate 5±0.25 V. Since the selected buck-converter is rated at 10 A but seldom reaches a quarter of that, its heat-sink doesn't get very hot, but the supplied standoffs were sufficient to separate it from the acrylic base to increase air-flow. CAVEAT: I added insulating washers to these standoffs, which were otherwise uncomfortably close to circuit-traces. This module's voltage should be calibrated before use (which involves holding down a button before powering-on), after which it can be configured to display both i/o voltage & output current.

The buck-converter was set to the maximum voltage tolerable according to the USB-standard, 5.25 V. The measured voltage had dropped to 5.2 V @ the exit to the switch before the Raspberry Pi, at the maximum expected load, having lost a little over crimp-connectors, 16 AWG wires & one switch. Then there's the Raspberry Pi's µB USB power-connector which was constructed from a short fast-charge (no data) USB-cable, with one end removed & the two exposed 18 AWG power-wires (the thickest I could find) terminated with crimp spade-sockets. The voltage-reduction resulting from a series of shonky connections is dependent on the current demanded, resulting in sporadic Low voltage errors from the OS (typically when the HDD were activated); so I increasing it to 5.5 V.

The selected buck-converter also permits one to define a current-limit, & 4 A (calibrated by shorting the output into a 0.5 Ω 50 W resistor) was found to be adequate to avoid activation in normal circumstances. This prevents excessive loading (& consequently heating) of either the PSU or buck-converter, should a short-circuit occur (CAVEAT: it won't prevent disaster if a nut drops from the standoffs above, through the cooling-vents in the top of the PSU, since this is upstream from the buck-converter).

Switches have been used to independently isolate the three power-connections. This facilitates sequential application of power to the USB-hub & then to the Raspberry Pi, so that the HDDs are available to mount when the latter is booting. The battery is also connected via a switch, because removing the crimp-terminal (for whatever reason) is annoyingly hard. The minimum requirement is satisfied by SPST switches, but the greater functionality of DPDT switches (which cost little extra) may facilitate re-use in a different project. Each switch requires a 12 mm cut-out, which exceeds the diameter which can safely be achieved using a twist-drill bit without cracking the acrylic; I used a step drill-bit.

User-interface

The Raspberry Pi is connected to a TV using HDMI. Since the TV's tuner isn't required, one might be tempted to replace it with a simple monitor, but monitors typically lack any audio-amplifier (see below), & the primary input-device is the TV's remote-control & infra-red receiver, which via HDMI-CEC can relay commands to the Raspberry Pi, avoiding any requirement for a dedicated infra-red receiver. CAVEAT: the HDMI-support on older TVs may not include CEC, or may need it to be enabled, & the manufacturer may refer to it using some fatuous brand-name (typically called "something-(link|sync)").

Audio

Perhaps just when playing music, or also when watching a film, the output from the TV's internal speakers doesn't quite cut it. Your new TV may have a greater pixel-density than the human (& owl) eye, but the sound is probably no better than from an old cathode-ray model; it's probably worse since the internal speakers typically face either backwards or downwards, to permit a oooh-so-stylish narrow front bezel.

One can connect an audio amplifier (& Hi-Fi speakers) to the TV's audio output (typically RCA sockets) rather than directly to the Raspberry Pi. The TV will probably have an item buried in its menu to manually redirect the audio signal, though mine switches automatically on sensing the presence of the connected audio amplifier. One could alternatively take the audio signal from the headphone-jack on the Raspberry Pi.

Status-LEDs

The depicted circuit is powered from the Raspberry Pi's 5 V GPIO-pin, & can supply a constant current (irrespective of the LED's colour) to any of four LEDs, each according to the state of a dedicated GPIO-pin. Individual trimmer-potentiometers are available to fine-tune the brightness.

echo 'nohup /storage/.config/show_status.py --poll --period=15 &' >>/storage/.config/autostart.sh	# Poll the h/w periodically.