Scientific Computing

Carriers in numerous countries worldwide have shutdown 2G networks to free spectrum for 4G and 5G services. Embedded modems such as automotive (OnStar) and alarm systems may again be impacted by these shutdowns. In developing regions 2G networks may linger for several more years, due to cost-effective legacy devices.

Those designing IoT and other embedded devices with cellular modems should consider LPWA 4G LTE, particularly Cat M1 and NB1 to help ensure global functionality. Each geopolitical region has unique LTE bands, but often OEM modules with the same pinout have region-specific models. OEM LTE modules will incorporate at least some bands for each region, so that global LTE roaming for even inexpensive LTE modems will become increasingly common.

2G shutdown dates from Digi

Notes

• 2G / 3G shutdown table
• 2G shutdown chart from Digi

The USA 1G AMPS and 2G D-AMPS networks were shutdown in 2008. In June 2013, Sprint shutdown the Nextel iDEN network for LTE band 26.

A local Git repo can become corrupted in rare circumstances, perhaps doing a git commit just as a computer crashes or loses power. A common symptom of a corrupted local Git repo is any Git command except for perhaps git diff giving error:

fatal: your current branch appears to be broken

The changes are likely still present, as seen via git diff. Previous commits that were not pushed to remote are likely present as well in the form of the modified files, but the historical local Git commit deltas may not be recoverable. This recovery will in effect “squash” the local commits that weren’t previously pushed to remote.

NOTE: work done on other branches that weren’t pushed to remote may not be retrievable.

Recovery

1. Copy the directory tree of the affected local Git repo, preferably on another hard drive or in the cloud.
2. git clone a fresh copy of the remote Git repo to a new directory
3. Compare folders to incrementally copy into the new directory the changes from the old corrupted directory. If there are a large number of changes, consider making the changes via multiple Git commits.

Notes

• git recovery reference

Missing LaTeX fonts can be added via TeXLive or MikTeX. Using the symbolic fonts is as easy as:

\documentclass[a4paper,12pt]{article}

\usepackage{fontawesome5}

\begin{document}
  \faGithub This is a GitHub logo.
\end{document}

This may need to use XeLaTeX. We use fontawesome5 instead of obsolete fontawesome, which is version 4. FontAwesome5 is in TeXLive 2018.

TeXLive is popular across operating systems. Linux users can use system package managers to install groups of TeXLive packages. Advanced Linux TeXLive users can independently setup TeXLive to get the latest packages individually, to save hundreds of megabytes of install space.

For marvosym:

Windows/Linux tlmgr:

tlmgr install marvosym

Linux: apt:

apt install texlive-fonts-recommended

For fontawesome5:

FontAwesome gives popular emoji and website icons.

Windows/Linux: tlmgr:

tlmgr install fontawesome5

Linux: apt:

apt install texlive-fonts-extra

MikTeX is another LaTeX distro on Windows. MiKTeX will automatically install packages as needed. If that isn’t happening, check:

Start → MiKTeX Console → Settings → General → “Always install missing packages on the fly”

Otherwise, manually search for package name and install:

Start → MiKTeX Console → Packages

The unofficial OpenCV PyPI wheels work with pip install methods:

pip install opencv-python

For ARM / Raspberry Pi:

pip install opencv-python

also works for certain ARM platforms like the Raspberry Pi.

OpenCV is trivial and fast to install on a Raspberry Pi via pip as described above.

For the latest extended functionality that hasn’t yet been incorporated into the core package, OpenCV including the Extra contributed modules may be obtained by:

pip install opencv-contrib-python

• OpenCV release notes
• Conda-forge OpenCV 3
• (reference) Windows-only OpenCV wheels. (cpMN where you have Python M.N). contrib includes OpenCV-extra packages.

If ... is not supported on this platform error be sure it’s using desired Python install. One may have to manually specify the path for the pip command e.g.

$Env:SystemDrive\anaconda3\scripts\python -m pip install opencv-python

We made several test scripts to try out the OpenCV install. Compiling OpenCV yourself allows customizing and optimizing OpenCV for your computer (e.g. using GPU, TBB, OpenCL, etc.).

The conda install opencv and conda install -c conda-forge opencv methods for OpenCV continue to be BROKEN for video/image reading and display. Use pip install above instead.

If YouTube won’t accept a video upload, or the video never completes “Processing” on YouTube, try re-encoding the video with FFmpeg. Sometimes lossy conversion is necessary to achieve the YouTube recommended upload settings.

Periscope video downloads use MPEG TS container with .ts file extension. These .ts files can be played back in VLC or similar to confirm content. A lossless conversion to YouTube is possible with:

ffmpeg -i pscp.ts -bsf:a aac_adtstoasc -codec copy -max_muxing_queue_size 1000 pscp.mp4

The FFmpeg option -max_muxing_queue_size is an arbitrary parameter. The default queue size is sometimes not large enough:

Too many packets buffered for output stream 0:1.

• Do not use .mkv extension for YouTube uploads, it will fail to process.
• certain formats like AV1 or FFV1 will fail to finish processing the upload
• after completing Uploading, video should start Processing in less than 5 minutes. If not, it will probably never actually work.

Uploads that don’t finish conversion processing after upload are not viewable on YouTube.

Generating a range of datetime data is a common data analysis and simulation task. Here we show examples of generating datetime vectors for Python datetime and numpy.datetime64

Python datetime

Python datetime is often used as timezone-naïve with UTC as the assumed timezone. This custom avoids ambiguities when working with Pandas and Numpy, which are foundational for Python data science.

Generate a range of Python datetime like:

from __future__ import annotation
from datetime import datetime, timedelta


def datetime_range(start: datetime, end: datetime, step: timedelta) -> list[datetime]:
    """like range() for datetime"""
    return [start + i * step for i in range((end - start) // step)]


dt = datetime_range(datetime(2019, 12, 1), datetime(2020, 4, 1), timedelta(days=1))

Numpy datetime64

Numpy datetime64 generates a range of times like:

dt = numpy.arange('2019-12-01', '2020-04-01', dtype='datetime64[D]')

Pandas has the date_range function to generate time vectors.

Matlab datetime works much like Python datetime, and is generally recommended. Matlab plotting functions generally support “datetime” class. It is sometimes necessary to manipulate Matlab plots involving datetime for the desired result.

dt = datetime(2019, 12, 1):datetime(2020, 3, 1);

dat = rand([100, length(dt)]);
y = 1:size(dat, 1);

pcolor(dt, y, dat)

If the plot doesn’t have the desired datetime axis formatting, try datetick.

datetick('x', 'yyyy-mm-dd', 'keepticks')

Generate vector of datetime. Python matplotlib supports datetime.datetime and numpy.datetime64 in most plot functions.

After updating an operating system graphics driver while Matlab was open, an error may occur upon plotting in Matlab like:

MATLAB has experienced a low-level graphics error, and may not have drawn correctly. Read about what you can do to prevent this issue at Resolving Low-Level Graphics Issues then restart MATLAB.

The solution to this issue typically is, as the message suggests, just restarting Matlab. A full reboot is not always necessary, but try that if the error persists.

Skipping a test with Meson or CMake by returning error code 77 is a de facto practice. Sometimes it is only feasible to know a test should be skipped by attempting to run that test.

Meson build system accepts return code 77 by default as a signal to skip the test. Configure CMake to skip return code:

add_test(NAME MyTest COMMAND mytest)  # arbitrary

set_property(TEST MyTest PROPERTY SKIP_RETURN_CODE 77)

The vcgencmd utility allows reading a few dozen hardware measurements on the Raspberry Pi boards.

CPU Temperature is checked by:

vcgencmd measure_temp

Typical temperatures in office environment, with case:

A red thermometer icon GPU-superimposed on the Raspberry Pi display output means the Raspberry Pi is overheating and is throttling the CPU and GPU to avoid self-destruction.

Raspberry Pi 0, 1, 2, 3 temperature thresholds:

CPU temp. [C]	icon	throttle
< 80	none	none
80 - 85		CPU
> 85		CPU & GPU

Raspberry Pi 4 temperature thresholds:

CPU temp. [C]	icon	throttle
80 - 85	none	CPU: 1000 MHz
85 - 90		CPU
> 90		CPU & GPU

The current Raspberry Pi CPU clock speed is obtained from

vcgencmd measure_clock arm

The output is in units of Hertz. The Raspberry Pi CPU clock speed and power consumption is dynamic:

Log temperature measurements with crontab -e. This can periodically log temperature and CPU frequency, e.g. add a line like:

@hourly  vcgencmd measure_temp | /usr/bin/logger

logger writes the measured parameters into the system log.

Related: Monitor Raspberry Pi DC input voltage